You just replace the mirror glass. To remove, adjust mirror to inner/uppermost position. Using a flat bladed screwdriver in the opening, carefully disengage the clip retaining the mirror to the base plate. Move the glass in a circular motion to disengage the remaining clips.
I've never had to do one, these are the directions from my Gregory's.
And once you have the mirror out I would take it to a glass cutter and see if they can cut you a piece of mirror the same shape or something. If that lucks out I would talk to windscreen repairers and ask them - this is a long shot