Just like that picture above shows.....on the bottom edge there is a small hole...it says to use a flat head screw driver...well it needs to be a very small one. but you just shove it in and it will release the clip that holds the mirror on. I did this when I had my windshield replaced a year or so back.
just to update this if anyone needed to do this, a coat hanger works just as well as a screwdriver. worked well enough to launch the remaining piece of the mirror to the trunk space :l