To change the deck drive belt on a riding lawn mower, remove the key, and disconnect the spark plug. Set the deck to the lowest position, and remove the deck belt guards. Remove the belt-keeper rod, loosen the nuts on the idler pulley, and remove the existing deck drive belt. Set the new deck drive belt into place, and reassemble the lawn mower.
To ensure the new deck drive belt is properly placed on the lawn mower, take a digital photo of the assembly before removing the old belt. Some mowers require moving the deck to the highest, rather than lowest, position to remove the deck drive belt. Refer to the owner's manual to determine the correct placement of the deck. Place wooden blocks under the deck after setting it to the appropriate height to prevent injuries.
After reassembling the lawn mower, start the mower, set the throttle to full throttle, and engage the mower deck. Turn the lawn mower off, and disconnect the spark plug wire. Set the deck mower to the recommended position, and check the belt to ensure it is working properly. If the belt is working properly and no damage has occurred, reconnect the spark plug, and use the mower as usual.