The rear view mirror glasses are held in place by two things: two sided tape and a spring. The glass will come away from the tape if you pull hard enough but first you have to undo the spring, which acts more like a circlip.
To remove this spring you'll need a flashlight and something long and thing and strong because the spring is a four inch circle with two hooked ends that go over mounting tabs on the guts of the mirror.
To remove it have somebody push the mirror back as far as it will go. Shine the flashlight in there and you'll see the spring and the hooked ends retained over some plastic mounts. Now "all" you have to do is poke something in there and unhook the two ends. It's actually not as hard as it looks which is probably a good thing as it looks hard.
Now you have to pull the mirror glass off the two sided tape. This takes some effors and if you want to re-use the glass be careful.
To install new glass put the spring in place except for the hooked ends, locate the glass in place then in the same way you undid the hooked ends, hook them over the retaining tabs. Done. Good as new.