Anyone know how to replace the deck belt on a murray riding lawn mower, part # 405002x8a?

Lower the deck all the way and remove the belt between the deck and the engine's pulley. Your deck belt is held tight by a spring loaded tensioner pulley which must be pulled back to create slack. Some manuals call this a swing arm, check your manual. It never hurts to have a helper do this for you while you peel the belt off the first pulley.