I believe you can access the belt by removing the powerhead upper casing. To do this, I flipped the powerhead over, and unscrew two screws located and the rear of the powerhead. Once the screws are removed, there are four engagement tabs (two at rear and one each side) that you can lift slightly, and remove the upper casing (remember to flip the powerhead back over to do this last step). The motor/brush/belt should now be accessible. Remember to check on the two motor filters (two foam filters attached to the rear of the powerhead upper casing) for excessive dust etc. Good luck. PS make sure the unit is unplugged -- safety 1st.
HL, October 2004