Hi, I have the manufacturer's notes for a Murray 20", which might help:
Warning: remove the cable from the spark plug.
1. Remove the belt cover.
2. Remove both front wheels.
actions 3, 4 and 5 refer to the side adjacent to the drive belt:
3. Remove the pinion gear retaining ring and the drive pin through the pinion gear.
4. Remove the dust cover and the pinion gear.
5. Remove the hair pin from the axle, which will allow the axle to slide.
6. From the opposite side, with the wheel removed, pull the axle across the front of the mower, such that the belt can be removed from the pulley on the axle (drive pulley). Remove the belt from the drive pulley
7. Remove the belt from the pulley at the other end of the belt(engine pulley).
8. Remove the belt from the middle pulley (idler pulley) by pulling the belt fully forward onto the idler pulley and then sliding the belt off the pulley and through theb gap in the pulley mounting bracket.
9. Fit a new belt by reversing the procedure.
Hope this helps.
Kevin Nurse, August 2007