Drop the deck onto some 2x4's then slide it out of the way.
Engage the parking brake.
Drain the fuel tank & remove the battery
Stand the mower upright on it's back wheels
Take a few photos for refference latter on.
Undo the idler pulleys ( bolts are captive ) slide them down far enough to get the belt past the keepers.
Check them all for free & quiet rotation if doubtful replace them.
Take the belt off the idlers then the engine pulley then the tranny pulley
There will now be enough slack to take the bel right off the front spindle and over the rear fan .
Replace in the reverse mannar.
