To replace the glass only requires tilting the mirror up and
removing a clip that holds the mirror in place. Removing the entire
assembly calls for removing the plate on the inside of the car that
covers mirror mounting hardware and removing the door panel to get
at the wiring.
I highly recommend obtaining a service manual. They are
available on eBay for around $10 and are invaluable if you plan on
doing any kind of service on your Subaru.