How do I change the bulb that lights the AC/Heater buttons?

If the glass globe of the bulb comes out leaving the metal bulb base behind, you may need to pull the whole thing. 1 screw holds the bulb socket support in place.
Those plastic button stems are not the same light conducting fiber optic type material we have today. They yellow with age too. New bulb more trouble than its worth ? Good luck

I partially cut the blue cover open on the bottom so more light could get to the buttons. Helped a little..