the mirror glass is easy to get off once you get the hnag of it

on the back of the mirror there is a round platic circle with 4 points on it
these locate into the coresponding holes in the mirror housing but there is a round locking ring which you turn a few mm to lock everything into place via the hole underneath

when they have removed the mirror, they have pulled the round locking ring off the housing so you can't just stick another mirror onto it as one of the adjusters is still left in the pic but the up/down one has gone with the ring because it's missing

let me see if i can find a pic to show you