How do I replace the carrier assembly on Black and Decker BR318 belt sander(the front roller assembly)?

Probably not.
It may have just had a part break inside, like the on-off switch, or one of the wires to one of the brushes, etc., etc.
I buy these regularly, and they are inexpensive, which also means they are made of inexpensive ( cheap) parts.
It wouldn't hurt to take it apart, knowing it may not be repairable. Sometimes parts are available on-line.
Be blessed.

The 7447 does not have a release lever. To change the belt, you must release tension of the rollers by pushing the front roller straight back until you hear it click and lock in position to change paper. Remove old paper and replace with new paper. Make sure if new paper has directional arrows to install properly.
Now you must release the front roller to put tension on paper. To do this push the front roller from the side that has the adjustment screw. This will release the front roller and allow it to go back into position. Be careful as it pops back quickly and with some force.
Turn on sander and adjust centering of paper with adjustment screw found near the front and side of sander.

To change belt:
1) Put front roller on floor and push down until roller locks back.
2) Put on new sand paper.
3) Loosen tracking screw until front roller is released.
4) With sander running, use tracking screw to center belt.

You came to the right place:) Here it is:

http://www.dewaltservicenet.com/documents/English/Instruction%20Manual/90511879,BR318.pdf

Hope this is very handy for you.
Regards, Farmerbear at FixYa.com

To change the belt, you must release tension of the rollers by pushing the front roller straight back until you hear it click and lock in position to change paper. Remove old paper and replace with new paper. Make sure if new paper has directional arrows to install properly.
Now you must release the front roller to put tension on paper. To do this push the front roller from the side that has the adjustment screw. This will release the front roller and allow it to go back into position. Be careful as it pops back quickly and with some force.
Turn on sander and adjust centering of paper with adjustment screw found near the front and side of sander.

Usually there is a lever which when released allows the two rollers to go closer together, thus releasing the tension on the belt. You can then slide the belt off the machine and put on the new one before re-tensioning using the lever. If this helps, please rate me! :)

I have one of those and I just smack the front roller ahead with a brass or plastic hammer. I hit the movable bracket that the roller is attached to. It takes a pretty stout whack to do it though. hope this helps you

This is a good sander but no portable belt sander is really a floor sander.

Check first that you have retensioned the belt... i.e. when you removed the other belt, you would have had to pull on a spring-loaded lever to remove the belt tension before you added the new one... make sure the tension lever is flipped back in to ensure the new belt is tensioned...

Next, and the likely problem you are having, is belt tracking... There should be an adjustment knob up near the front of the sander near the front roller, either on top of the front part of the sander, or to the side near the front roller... When you turn this knob, it makes the belt track either left or right over the sander's base. This knob will allow your belt to run square and true and not spin off to one side...You need to adjust the tracking using this knob while the belt sander is running and with the new belt tensioned and in place, so turn the sander upside down, and power it on with one hand, then turn the knob with the other hand whichever way is needed to have the belt track square and in the middle of the rollers. Make small adjustments on the tracking knob to begin with. They can be sensitive at times.

That should stop your belt spinning off to one side.