Remove the belt cover guard from the tiller by removing the screws that hold it onto the tiller housing and then place the cover off to the side. Removing the cover exposes both drive belts.
Locate the reverse belt. The reverse belt is the shorter of the two belts. It runs three pulleys and it rests higher in the pulley housing than the forward belt.
Press in on the idler pulley for the reverse belt to increase slack on the belt. The idler pulley is black and is the smallest of the three pulleys.
Pull the reverse belt off the three pulleys and discard it.
Locate the idler pulley for the forward belt and press inward to increase slack on the belt. The idler pulley for the forward belt actually has two pulleys grouped together.
Pull the forward belt off the pulley system and then place the new belt onto the pulley system. Make sure you route the belt correctly. If needed, make a diagram to ensure you route it correctly. The forward belt is more confusing because it has more pulleys.
Push in on the idler pulley for the reverse belt and then place the reverse belt onto the pulley system.
Place the belt cover in place and then secure it with the screws and screwdriver.