Cover the mirror with packing tape to prevent shattering
Begin to pry it away by sliding shims between the wall and the mirror
Place a shim beneath the hook of a pry bar (protecting the wall) and begin to move the bar behind the glass, maintaining pressure and listening for the paper to tear. It will release in increments.
Once at the bottom, it will need support to prevent a fall, and this may mean two people are required for success.
Remove it and carry it away.
Then begin repairing any damages and prepping the surface for a new installation.
You may have to fill any holes in the drywall, and wait for that to cure before installing the new mirror