There are 2 allen screws on the rear of the machine. Use an allen wrench and access the side in which the walk belt is tracking towards. (tracking to the right go to the right screw) Insert the allen wrench and rotate the screw one revolution CLOCKWISE. This should move the belt 1/4 inch towards the center. If needed you may rotate the screw one and possibly 2 more revolutions but more than that you may have another issue with the machine.
Good luck! Tim