Jay,
If you buy a new mirror replacement glass you will see what the process is. If I remember right, the old glass must be pried off. The new glass has a round center clip assembly that must then be snapped into the mirror housing.
Pretty straight forward.
Some automotive glass shops will cut you a new mirror for alot less then the factory replacement, but then I believe you have to glue the new one in.
Mike J.

 

The Land Rover Part is a whole mirror assembly, had to replace my passenger side one a couple of years ago when a truck clipped it travelling in the opposite direction. Lost the mirror but the rest was ok, still had to buy the lot though, although was relatively cheap from what I remember < $50