Don't break your mirror glass. Bend a slim phillips head screw driver just enough to clear the mirror to access the screws that hold the adjusting mechanism to the outer housing. I've removed several mirror plates this way for painting. You may have to spray some penetrating oil and let it sit before attempting this. These are after all 50 years old in most cases! You DO NOT want to strip the head of the screw! Take your time and use steady pressure. Good luck.