The rear of the glass has a rotating locking ring, which you lock via the little slot
Poke a slim screwdriver into the slot so the blade is 90 degrees to the glass and lever it either left or right and it will unclip the mirror backing from the housing
If the motors have been pulled too much though they tend to strip the plastic adjusters which means a new unit or dismantling a spare for the job
It is very easy once you have done it once so don't fret