Unscrew the bezel and the mirror literally falls out the mount.
It's probably a 20mm gold plated mirror like the ones from lightobject.com, the same ones im running in my 1.0 buildlog laser.
The mirror can crack if its been dirty for a while, the dirty mirror absorbs the heat instead of reflecting it like a clean mirror will, so always make sure the mirrors are kept perfectly clean.
You can make a new mirror out of an old pc hard drive disc platter which will be superior to the cheap gold plated mirrors, I have a set waiting to be installed in my machine check out viewtopic.php?f=14&t=1000
for all the details.