Really easy to do mate.
TILT YOUR MIRROR UPWARDS AS FAR AS IT GOES.
put your fingers in the gap between the mirror and the mirror casing and pull it off.
try spreading the pressure by using 4 fingers from each hand as you'll snap the glass if you pull too sharply or put too much pressure on one point.
I done mine a couple of weeks ago when I changed my mirror cases, I found once I got the bottom of the mirror off the rest just pulled staright off without any drama.
Good luck, just bear in mind to spread the weight as they are fragile.