Please tell me how to change the drive belt on an MTD lawn tractor.

well, you will have to remove or at the least loosen and pull the pto pulley down enough to remove the old belt, while you have the brake set. Then you can remove what pulleys you have to to get around where there are belt guards. Have a diagram, or draw a routing before removing the old belt. Some units have a variable speed pulley and have more than one drive belt.