Think it is just a case of pull and hope! On the back of the mirror the is a black plastic clip (it's an industry standard type) which sticks onto the glass.
My advice for retrofitting is to put the clip back into the wing mirror, then stick the glass on. I tried to do it on the table, it was off by a couple of mm, so when I put it back it, the edge of the mirror caught on the frame when I moved it.
Angle the mirror down as far as it will go and pull/lever sharply on the exposed edge. You think it will break, but it should come clean away with its clip. As Giblets says, its important to accurately re-mount the clip on the new glass or it will foul the surround when you come to adjust it.
Safest bet is to go for the correct OEM mirrors, I wasted money twice on so called replacements from eBay, they're either slightly the wrong shape or too concave. Ended up paying £104 for a genuine pair!