Maybe these instructions will help more:
The cover that goes around the mirror stay just clips in place. Carefully pry it off.
Once that is removed, you can see a screw in the bottom of the mirror stay - the lock bolt. Turn that counterclockwise to loosen it. Even if it's really tight, you have to loosen it to get the mirror stay off the lug, which is the part glued to the windshield.
Once the lock bolt is loose enough, you can slide the mirror assembly towards the nose of the car, off the lug.
If everything goes as it should, the lug will still be glued to the windshield, ready to accept your new mirror.
When you slide your new mirror onto the lug and tighten the lock bolt, the springy stuff inside the new mirror stay should clamp onto the lug.
Good luck!