Use a small flat screwdriver up in the tiny squarish hole on the bottom of the mirror mount. Push up slightly and wiggle it a bit, it will slide off. Watch out for the windshield it may pop out quick-you dont wanna scratch it. I taped some cardboard around mine just in case, not necessary if you have a steady hand I would think. Once you do it once or twice you will get the "feel" of it. Don't pry it!
If its stubborn try wiggling it. Remember, when in doubt wiggle it out.