Answer:
There are two types that I know of. One has bar in both the transmitter and opener with a series of little switches on it. They can be set in any combination of on/off as long as they match. The other type only has this in the transmitter. You set the combination you want and on the opener where the wall button wires attach there should be a small 1/4 inch square button. Push it and it should flash and then stay on. Push the button on the transmitter and the button should flash and go out. The opener should receive the new code.