Remove the plastic shrouds that cover the blade pulleys from the deck itself.....you'll see the belt in all its glory, complete with the brake system for the pulleys which need to be pulled out of the way.
Underneath, accessing the large crankshaft pulley beneath the engine, you'll see guide rods that run close to the pulley to stop the belt from popping off. Slacken them but don't remove.
At this point you should be able to release the belt but if its too tight you may have to release the cable that tensions the blade belt by pulling on another pulley, which allows the deck to move forward a bit.
Use a genuine belt - they cost an arm and a leg but they are the best choice.
John
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