Turn the mower on its side to access the belt cover, which is underneath the mower.
Remove the belt cover screws on the top side of the mower, then remove the cover.
Move the belt guide away from the driver assembly by removing one of the screws that hold it in place and loosening the other.
Remove the damaged or worn drive belt by first loosening it from the pulley, then pulling it from the bracket.
Run the replacement belt through the bracket, around the pulley.
Push the belt guide back into place, replace the screw you removed and tighten both screws.
Replace the belt cover, and secure it with the screws on the top side of the mower. Stand the mower upright again.