Most altimeters can be adjusted externally. There's a little screw adjacent to the knurled knob for setting the Qs. Undoing the screw releases the locking mechanism then twiddle the knob to set the altimeter and then tighten the screw.
Whoa there....
Though there is an external adjustment for the barometric scale on altimeters, I'm sure that such adjustments are a "maintenance" activity, and require a qualified maintenance release.
If the person with the screwdriver has satisfied themselves that they are entitled to do this work, they should be very weary, if that screw falls out, a part will drop off inside the altimeter, and then it'll have to be disassembled for repair.
If you have succeded in adjusting the altimeter subscale, you probably have not solved the real problem, which could be an adjustment of the encoder is needed. Even if it is an encoding altimeter, you still have not solved it.
The answer to the original question, which I thought would be self evident, is that the aircraft has to go to a qualified instrument/avionics shop. I believe that even so equipped amature built aircraft do, as the air traffic control system does not care who built the aircraft, if it is controlled airspace sqawking an altitude!