Ha, I just did this 2 weeks ago.
You slip a flathead screwdriver into that little divot in the bottom where the mirror meets the glass. You slowly tilt the screwdriver forward, and slowly pull the mirror junction upward.
to put the new mirror in, you will need a small hammer and a piece of soft wood. you can slip the new mirror on halfway, but you need to put some muscle in to get it all the way on. so you position the piece of wood at the top of the mirror junction, and hit that wood with a hammer to drive that mirror into position.