I haven't tried on the focus, but on other models they can be easy or hard. There are little clips on the back side that hook into the mirror. Sometimes you can give it a good solid tug and it comes right off. Sometimes the clips will break and if so you can just get a good epoxy and put a little on the back. If the mirror glass come off then its real easy because you can get behind it and pry the clips up.
2013 Oxford White ST3
2012 Sonic Blue Hatch ---> Sold
|FSwerks Stealth Catback||Steeda Cold Air Intake|
2003 Dark Shadow Grey ZX3 ---> Sold
2000 Silver Frost ZX3 ---> Totaled