Hello did this one some years ago (removed and replaced the plastic outer only)
It took me about a hour to found out how to do this, the mirror section is held in by two interlocking clips, that you can get at this by first placing a screwdriver into the side of the mirror section (side that faces the door)
Remove the mirror, unscrew the motor section, and you can then see how the outer section is made, the overall case will slip off the main support, re assemble in revers order, place the mirror section back in starting from the side that faces away from the door, push the mirror in (be carefull not to use too much force) or you will snap the final interlock clip.
You could go down to a scrappy and have a look at a damaged one, just to get a feel of the job
Regards Dutch RR ( Now Freelander)