To remove mirror glass you'll need a flat head screwdriver.

Push the base of the mirror down at the point 'X' in the pic
Then place the Flat head screwdriver behind the mirror glass as the top around the area that the arrow points to in the pic.

Now push the screwdriver away from you to level the mirror glass towards you. (NB, I've not yet managed to brake any mirror glass doing this yet, but it could be possible, so do not proceed if you don't want to take this risk/don't have a replacement).

The glass should then unclip and fall forwards.