On the W210, you push the mirror housing backward - ie toward the back of the car. It will hinge around the A-pillar, it takes a lot of force but you won't break it. Once you've pushed it back, if you look inside the housing you'll find a small retainer clip (sprung). Insert a screwdriver into the housing and undo the clip, let the mirror spring back, and then simply slide it off.

Its a bit tricky but shine a torch in there and you'll see the clip. It will be much easier if you get someone to hold the mirror while you undo the clip.

Once the housing is off, the method of removing/installing a new mirror will be self-explanatory. Be careful not to break any plastic lugs.

This website will show you how it works:

http://mb.braingears.com/W210/w210/disassembly_88hoodfend.htm