Lawn mower belts transfer energy from one rotating pulley to another. The belts must be tight to transfer this energy, so each belt includes some sort of tensioning mechanism. Belt guides also may keep the belt in the right place between pulleys. Changing lawn mower belts involves locating and loosening the tensioner mechanism, removing the old belt and replacing it, and tightening the tension again so that the belt can work properly.
Remove the Old Belt
Remove any guards or shielding around the belt to be removed. Locate your tensioner mechanism.
Loosen the tensioner mechanism so that there is no tension on the belt. For a cable-and-spring combo, look for the keeper, which will be a bolt or clamp or bracket holding the cable in place. Loosen the bolt, release the clamp, or remove the cotter pin from the bracket. For a spring-loaded pulley, pull the spring off the idler arm and tension will be released. For the sliding mechanism, locate the nut and bolt that clamp down the sliding component. Use the adjustable wrench to loosen the nut to release the component and slide it until tension in the belt is released.
Feel the belt to see how much give you have. If the belt is still tight or just seems stuck on something, check for hooks on the belt guides that may still be holding the belt in place.
Lift the belt off the belt guides and the pulleys once the tension is loosened.
Install the New Belt
Put the new belt onto the pulleys and belt guides. Make sure it is straight and untwisted.
Tighten the tensioning mechanism by reversing the process you used to loosen it.
Pull the belt slightly to check the tension. It should resist and stay in place. If it loosens, tighten the tensioning mechanism more.