Ok guys. I think I can assist. Given the date, it may be like mine.
The control box, which is usually located in the garage but near the outside panel, has a small slide switch located at the top of the pc board (pop the cover off-its where the switch and battery are).
Slide the switch which puts it in program mode. now lift the cover of the panel outside, press in the new code, and slide the switch back to it's orginal position. you when you're in program mode, because the red LED flashes.