How to remove the plastic belt lift knob?

Make sure the knob on the side of the main floor nozzle is engaged. This is a common problem. Check the brushroll belt for tension. Put your palm on the brushroll as it is spinning. You should not be able to stop it. If you can stop it the belt is stretched and worn out.
The belt is attached to the clutch assembly which must be removed. There are 3 torx screws that have to be removed to gain access to the clutch assembly. Two are visible and one is hidden. I would leave this to professor to remove. It does take some prying to remove without breaking the plastic that surrounds it. The clutch assembly can be taken apart (again professional) and the belts replaced. Or install a new clutch assembly if the previous one is making a loud noise when running (engaged).
New belts take a bit of stretching to attach to the motor axle and the brushroll.

First remove the bottom rug plate. Some have screws, but most have tabs holding it in place. The upper plastic cover must also be removed. Pull off the rug heigth indicator knob first. The upper cover is held in place by 4 tabs along it's outer sides. Look for little arrows in the base that indicate the location of the tabs, and pop them loose with a screwdriver. The upper cover should now lift off. Place the new belt ( # 38528040 or package # 40201190 ) around the brush assembly and put the brush back in. Now you must hook the new belt with a strong screwdriver and stretch it up and around the motor shaft. This will require some strength, as the belt must be stretched a considerable distance. Turn the brush with your fingers prior to replacing the cover, to be sure the belt is centered, and not caught between the brush and the base housing.

I have had problems with the belts on my Mach 5 overheating (smell of burning rubber) and also with the motor overheating and shutting down.
I learned the hard way that one of the issues might be that the roller/brush on the bottom of the unit has a metal bearing inside the roller/brush that can become clogged with hair (both human and pet hair) which effects performance of the unit.
Another item to check is if your dust filter is not plugged up which is located behind the removable dirt cup. This is the cup that you remove that collects the dust/dirt/hair from your floor. Take the removable dirt cup out by depressing the gold button and then look behind where the dirt cup sits for filter. This can be cleaned out by washing it with soap and water in the sink but make sure you let it dry overnight before placing it back in the machine. This probably causes some of the overheating when not enough clean air is passing through the machine.
It’s fairly easy to fix.
Lay the vacuum on the floor so that you can see the roller/brush. Then remove the phillip head screws that hold the protective plastic plate over the roller/brush. (There are about 6 screws that hold this plastic protective plate in place) Also, you will be removing the (1) screw that also holds the plastic plate over the belts. There are (2) belts that you can remove and when in doubt replace these (2) belts for around $9.00.
Once you have the protective plastic plate off that covers the roller/brush disconnect the belt is around the roller. Take the roller and cut off any hair that is wrapped around the roller. Next, grab the roller by each end and twist the ends caps and (1) of the ends will unscrew. Once you have unscrewed one of the ends remove any hair or dust that is trapped under this cap. (Be careful not to lose the small metal washer that sit on top of the bearing)You will notice under this cap is a metal bearing that can become clogged and jammed with hair. The hair should come out fairly easy and allow the brush to spin fairly easy once you have screwed the end caps back in place test that it spins freely. If it doesn’t spin freely replace the roller/brush with a new one for about $23.00.
When you put the belts back on the thick rubber one (Belt # 40201-271) has probably become stretched and should be replaced for about $3.00. The small diameter belt that goes over the roller/brush probably does not usually need to be replaced and does not usually fit very tight. I have heard that people will take the small roller/brush belt and turn it upside down to get a better fit.
I just did this on my Mach 5 and it runs like a new machine. I did end up replacing the roller/brush head.
Hopefully you will find this useful.

The Altima line has 2 screws on the hood (part where the knob is that says high carpet and low carpet) that you remove with a philips head screwdriver then you just lift the hood of the vacuum

Are you talking about the large power head (ised for floors) or the smaller power head (for upholstery)?

If you are talking about the large one the first thing I would check is the belt (accessed by turning the tiny lock knob on the bottom with a flathead screwdriver).

If the belt looks ok the next thing I would check is make sure an accumulation of hair hasn't built up on the ends of the roller....take the roller out and turn the roller by hand holding onto one of the ends...it shoudl move freely....

release the handle on the vacuum until it rests flat, then look at the part of the machine that has the hieght adjustment knob on it, that is the hood of the vacuum. there are usually three screws that hold it on. There should be a screw on the left and the right of the knob (towards the sides) and then the third is near where the small hose sticks out of the hood. once those are out the hood lifts up you will need to pull up on the brush roller ( you might need to use a small flat head screwdriver to pry it out, place it between the end of the brush and the housing and pull ) then take the new belt and place it over the small metal shaft that sticks out from the motor then put the belt on the roller and pull it into place dropping the roller back to where it was. also try turning the roller by hand to make sure you didnt pinch the belt under the roller.

Remove sole plate and beater bar.
Remove control knob by prising between yellow plastic side of knob and casing using wide bladed screwdriver.
There three torx headed screws holding the clotch assembly in place. Two are visible but the third is accessed via a circular access holein the main casing.
When these three screws are removed the clutch assembly can be pulled out. the thinner belt driven by the motor needs easing over the motor shaft.
There are two screws holding the end plate onto the clutch assembly. remove these, twist the end plate and remove. note where the fibre clutch plate and the metal plate fit and replace this belt.
On opposite end prise off spring washer and this assembly comes off the reveal the belt.

Refitting is the reverse.
Have a look on you tube and type in replacing clutch on Dyson DC4 as there is a 10 minute vidio that is quite good!!
Hope this helps!!!!!

1. Raise the vacuum head all of the way up. 2. Remove the vacuum head by removing the belt and releasing it. 3. Place the shampooer head on the vacuum by securing the clamp and them put the belt on 4. Take the bag off of the vacuum by twisting at the base just slightly outward and lifting upwards and unclamp it from the handle 5. Put water and cleaner into the water resevoir. 6. Put the reservoir on to the kirby where the bag was by holding it parallel to the vacuum then using both hands, twist it outward slowly until it goes down onto the unit then twist back to parallel position. 7. Put the coil looking piece on the shampooer head on top where the hole is...it will only fit one way successfully. 8. Put the other end on the under side of the water reservoir. 9. Lower the shampooer head all of the way down on the carpet just like you would the vacuum head 10. Turn on the vacuum and reach down and turn the knob at the top of the reservoir, this will begin putting cleaner (tons of foam) onto the carpet. Just work in 3' x 3' sections, turning off the foam intermitently until you are finished. I hope this helps.

on the kirby head is a square knob with the kirby name on it. use your finger and pull out the belt and rotate the knob until the metal pawl engages the belt. keep turning until it stops. the belt should be in the correct position to install the head. place the whole head onto the lower two tangs and lift up. now rotate the wedge clamp until it stops. then rotate the belt lifter till the knob is straight.

On the front of the power head is a knob with the KIRBY name on it. Rotate this knob in a counter clockwise direction and it will "lift" the belt off the motor spindle. Release the head lever, only one direction it will turn and remove the head. Turn the head over on its back, turn the KIRBY knob to release the old belt. Remove the brush guard by turning the two release fixtures. Remove the guard. Pry the brush out of the head. Remove the old belt. While the brush is out remove any build up of hair and lint off the ends that's where the bearings are. put a few drops of 3 in One oil on the ends. Put the new belt over the brush and re-install the brush into the head. the brush will go in only one way. With your finger pull the belt into the housing and rotate the KIRBY knob until the pawl inside grabs the belt. Re-install the brush guard by engaging the two metal pieces into the front of the head and pull down backwards over the head and rotate the holding clips back horizontal. re-install the powerhead on the machine, lock it in place and rotate the KIRBY knob clockwise. You should be all set!