Not difficult really.
Remove the blade.
Remove the belt guard (two cross head screws in recessed hole.
On the differential unit (the square plastic box in the middle of the axle) press down on it to release the belt tension. Belt can now be removed from the drive pully then it can be taken off the blade pully.
Often when drive stops the belt is not broken but rather, come off the rear pully. To put it back on press down (rotate the differential) to provide enough slack to fit the belt. Make sure the belt is in the Vee of both pullies.
For a loose belt it can be re-fitted without the need to remove the blade as the belt cover will move just enough for access
Mike, August 2009