Remove pedal that is pressed on to crank arm.nut is already removed...they are pressed on to crank arm.

You will need to remove the shrouds on wither side of the Exercise bike. You will see on each crank, or at least one of them, there is a cotter pin with a bolt The cotter pin needs removing and you will be able to turn that pedal arm to 180' to the other one. Spare parts appear to be obsolete, but you should get parts for the crank arm from a good cycle shop. Very often the cotter pin needs to be filed to fit. Tightening the nut locks it in place. There are two videos, the second one refers to cycles which is where my experience comes from.

https://www.manualslib.com/manual/2258999/Diamondback-910ic.html#manual

it is left hand thread

It is according to what your crank design is.
1st. design. This is a solid crank. Look at your crank. One side will have a nut, and a race with slots in it. If this is yours then you take off the pedal on the side of the nut. Now take off the nut, use a screwdriver to remove the race. Pull that bearing out, then push the crank through the opening, removing the other bearing. If you have a multiple speed bike you have to remove the sprockets first.

2nd design. After removing the sprockets there is a bolt holding the crank arms on the shaft that the bearing are on. Remove the cranks and then remove the bearings the same as above.

To remove the crank (the arm the pedal is attached to), you'll need a special tool. I got mine at a bike shop. (Bike shops are our friends.) I use the Park Tools CWP-7. It's pretty much a big bolt with a smaller bolt going through it. You'll remove the cap covering the spindle bolt (if it hasn't fallen out already), remove the bolt that holds the crank are onto the spindle, then thread the "big bolt" part of the tool into the threads for the cap on the crank. You'll then screw the inside bolt into the big bolt, which will push the crank arm off of the spindle.

I recomend you to disassemble again, and repeat the steps. Something must be there ARE not according with the instruction assembling. Try however for to see the some loose parts, bolts, nuts, and lube.
See this small instruction.s
God bless you
Remove the screws or bolts securing the ends of the pedals to the crank axles. Slide the ends of the pedals off the axles. Remove any screws or bolts securing the left and right disc shields to the base unit. Pull or pry the disc shields off the base unit and set them aside.

2

Remove any screws or bolts securing the left and right side shields to the base unit. Detach the side shields and set them aside. Examine the interior of the base unit and locate the drive belt.

3

Pull the drive belt free of the crank pulley, flywheel and any other pulleys. Discard the old drive belt and install a new belt in its place. Turn the crank axles by hand to confirm that the belt will turn smoothly when the device is in use.
Lubrication:
Detach the ends of the pedals from the crank axles. Spread a layer of machine grease over the crank axles, then reattach the ends of the pedals.

2

Remove the bolts connecting the pedals to the swing arms. Detach the pedals from the arms, then spread a layer of machine grease over the axles on the swing arms. On some models, the bolts may also serve as the axles. Reattach the pedals to the arms.

3

Remove the bolts connecting the swing arms to the elliptical's upright. Detach the swing arms from the upright, then spread a layer of machine grease over the axles on the upright. Reattach the arms to the upright.

Detachthe ends of the pedals from the crank axles. Spread a layer of machine greaseover the crank axles, then reattach the ends of the pedals.2
Removethe bolts connecting the pedals to the swing arms. Detach the pedals from thearms, then spread a layer of machine grease over the axles on the swing arms.On some models, the bolts may also serve as the axles. Reattach the pedals tothe arms.3
Removethe bolts connecting the swing arms to the elliptical's upright. Detach theswing arms from the upright, then spread a layer of machine grease over theaxles on the upright. Reattach the arms to the upright.

Sounds like one or both of the crank arms are loose. The crank arms are the aluminum bars about 10 inches long that the pedals are attached to. They are attached to the bottom bracket spindle by way of a square hole that is tapered so as to fit tightly as the nut on the end of the spindle is tightened. You can test this by trying to move the two crank arms in opposing directions. To tighten, you probably have to remove a dust cap to get at the nut. But if you don't have a wrench to fit, this is a 1 minute fix that any bike shop would probably do for you for free. To see this, Google images for --- bottom bracket crank arms --- good luck vote if you found this helpful Al K

Be advised the left pedal is left-threaded. Get yourself a nice Park brand pedal wrench.

What fixing nut?

How to:

http://www.parktool.com/repair/readhowto.asp?id=83

Yes, the crank-arm could be replaced.
1.You would need to remove the flat nut holding the crank to the frame. And then take out the bearing.
2. A special open-ended wrench is required to remove this nut. Please visit your local bicycle shop for a new crank-arm, and the required tools, also.
3. Put in new crank-arm and its bearing.
4. Grease the bearing.
5. Tighten nut.
6. Give it a test ride.