how do you change a broken dryer belt on a whirlpool dryer?

ALWAYS UNPLUG YOUR DRYER BEFORE ATTEMPTING TO DO ANY WORK ON IT.
If you have to check/replace/repair your drum motor,drum seals,belt,rollers, etc.
If replacing drum seals be sure to use a good grade of weather proof cement to hold the seal/s in place, some seals are furnished with the cement to glue it in place.
Note: This method works for most front loading machines.
Remove the electrical console that holds the timer switch,start switch, etc. from the top of the dryer if necessary. Be real careful not to knock any of the wires off of the timer switch or other electrical connections.
Remove the lint filter and the two screws that hold the filter housing to the top of the dryer lid.
Then remove the top of the dryer by inserting a screwdriver between the top and frame on each side of the dryer to pop the lid up from the retaining clips.
Note: Some of the tops tilt backwards on a type of hinge and do not have to be removed completly.
Remove the front of the machine by disconnecting the door switch and removing the two screws located on either side of the dryer frame.
Move the door and front of machine forward to remove.
This will expose the belt,idler bracket and drum, remove the drum and belt and this will expose the drum rollers, idler arm bracket with pulley, and motor with pulley. Pay attention to the route of the belt, this will come in handy when you go to reinstall the belt and drum.
Sometimes the belt will slip off of the idler arm or motor causing the drum not to turn.
Note:The way the belt goes back on is, as the belt comes off the drum has to be doubled (pushed together to form a loop) and goes under the idler pulley then to the motor pulley.

While you have the drum out, I would recommend that you lubricate the drum rollers at the shaft of the roller with a good grade of machine oil like 3 in One oil or a vasoline type of lubricant. This will save you a problem later with the rollers getting noisy, also check your drum rollers for flat spots, if they have flat spots on them this will cause a thumping type noise. You will need to replace the drum rollers if they have flat spots on them.