How do I fix a walking belt that is off-center?

First, remove the key and unplug the power cord. If the walking belt has shifted to the left, use the hex key to turn the left idler roller screw clockwise 1/2 of a turn; if it has shifted to the right, turn the left idler roller screw counterclockwise 1/2 of a turn. Then plug in the power cord, insert the key, and walk on the treadmill for a few minutes. Repeat until the walking belt is centered.