All the other answers are wrong. You get a wrench with the sander. It's thin so it fits between the pad and sander to hold the nut in place while you unscrew the pad.
There are a number of reasons why a belt sander will fail to track -
Poor quality belts.
Physical damage (dropped).
Uneven wear of rubber drive roller.
When one becomes troublesome it can take a deal of sorting. Best idea if there is no obvious reason is to start by trying an OE belt.
Make sure that you have a good set of brushes in there. If so and it continues to happen then the motor is no longer drawing the amount of current it needs to function properly, because it should not slow down like that.