Turn your Troy-Bilt self-propelled lawn mower onto its side, then lower the deck lift lever from the right side of the mower.
Locate the belt underneath the mower, then remove the belt guard screws using a Phillips screwdriver.
Pull out the belt holder rod with your hand, then you will be able to access the belt tensioner, which is medium-sized metal bolt beside the belt with a square-shaped hole in it.
Insert an Allen wrench into the square hold of the belt tensioner. Turn the wrench clockwise to tighten the belt tension.
Turn the wrench counter-clockwise to loosen the tension of the belt. Place the belt holder rod back over the belt once you have reached the desired tension for your belt.
Reinsert the belt guard screws to the belt holder, then place your back into the upright position.