Disengage the PTO lever to stop the mower blades from turning and lower the deck to its lowest position with the deck lever. This maximizes space between the deck and the tractor.
Find the belt guards on the mower deck. Each of the two belt guards attaches to the deck with hex screws. Remove the hex screws with a socket and remove the guards from the deck.
Find the pulley mounted to a spring-loaded bracket. This is the tensioner pulley. Push the tensioner pulley away from the belt to release tension on the belt. Pull the belt off the mower deck. If needed, draw a diagram of the belt before you remove it from the deck.
Place the new belt onto the pulleys on the deck. Push in the tensioner pulley and place the belt over the tensioner pulley. Let go of the pulley to restore tension to the belt.