tilt the mirror to gain access to either the top or bottom of the glass
there are 4 clips that are roughly at the following positions :-
11 o'clock, 1 o'clock, 5 o'clock & 7 o'clock
you can look at your new mirror glass to see the exact location of these clips
use a trim removal tool or something blunt and place between
the plastic mirror backing and the mounting plate on the motor assembly
and gently pry the 2 apart