Do the whole thing. You will probably find it easier to do if you take it down from the cieling. Loosen then adjusting screw on the belt by the trolly but be careful not to take it all the way off. The take the 2 screws off that connect the rail. Take the trolley apart and replace. Making sure to put trolly back to its same position, or risk having totake the motor cover off to correct the open & close limits. You can find it at a sears service center or any large garage door company. Or online.