[stepson]

I suppose you're talking about the illumination bulb in the AC control head? If so,
you will need to remove the control head. Unscrew the top half of the clam shell contruction and remove the top half by unclipping it from the bottom half. You will see two black wire-looking things that combine into one near the middle of the AC head. These are the fiber optic cables to the slider knobs. Behind that is where the bulb is located. Remove the screw holding the gear inplace and remove the gear noting the positioning of the gear. The bulb is press fitted into the housing. You may have to pry up a small wire that impedes the removal of the bulb. Remove the assembly and replace the bulb. It is a small bayonet base bulb.
Reassemble and plug in the AC head. Turn on your lights to make sure you got everything back in position correctly. If everything works okay and looks good, finish your reinstallation.