What should I do if the walking belt is off-center?

Remove the key and unplug the power cord. If the belt has shifted left, use the hex key to turn the left idler roller screw clockwise 1/2 turn; if shifted right, turn it counterclockwise 1/2 turn. Be careful not to overtighten. Then plug in the power cord, insert the key, and walk on the treadmill for a few minutes. Repeat until the belt is centered.