Disconnect the blade deck from the bottom of the mower. Remove the belt from around the engine pulley, the pulley connected to the bottom of the mower. Use an adjustable wrench to remove the bolts along the side of the deck. Disconnect the springs that are now holding the deck in place.
Remove the old belt from around the pulleys keeping it in place.
Place the new belt around the pulleys the previous belt was on.
Reattach the deck to the mower. First, reattach all of the springs keeping it in place. Screw all the bolts into place, then fit the belt around the engine pulley.