Most home treadmills are rarely serviced, but like other machines, they occasionally need tune-ups. The most common problem---when the belt of the treadmill begins to slip off-center---is one you can easily fix with basic tools.
Understand treadmill terminology. The belt is the rubber surface that moves underneath you as you run. The deck is the waxed wooden platform underneath the belt that gives support to the belt as you run. The front roller, in between the deck and belt on the front of the treadmill, is connected to the treadmill motor. The rear roller is a fixed roller between the deck and the belt.
Locate the rear roller adjustment screws, which are typically next to the rear roller. Most manufacturers use hex bolts. Get the proper sized hex wrench or screwdriver.
Turn on the treadmill to the slowest speed and determine where the belt is running. It should track directly down the center of the deck with an even amount of space on either side of the belt. Use the wrench or screwdriver to adjust the bolt on the right hand side of the rear roller. While the belt is moving slowly (0.5 mph), turn the bolt 1/4 turn in the direction that you need the belt to travel in order to bring it back to center. The belt will begin to creep in that direction. You may need to turn the bolt more than 1/4 turn if the belt is far out of alignment.
Once you have centered the belt, turn the bolt back slightly in the direction in which it started to stop the belt from moving across the deck. You may need to try this a few times to find the exact center and to stop the belt from creeping too far to one side.