Park the MTD mower on a flat, even surface. Engage the emergency break.
Use the controller to lower the mower deck as far down as possible.
Loosen the tension bolt, using the appropriately sized socket. Push the spring-loaded arm forward to release tension on the old belt.
Unwrap the old belt from around the drive pulley and the PTO pulley. Place the new belt around the pulleys according to the manufacturer's instructions for your MTD Yard Machine.
Pull the spring arm back to tighten the tension. Tighten the bolt.