I had the same problem. You need to take the front panel off, but disconnect the mains supply to the system. When you remove the panel the alarm will sound so pull one battery connector off to stop it. You can remove the keyboard from the panel but be careful it is fragile. The keyboard can be taken apart and the buttons can be cleaned with contact cleaning pads. Refitting is the reverse. Please be careful as if the system is old, the plastic bits will have become brittle. Good luck.
AndyandPam, December 2019