The mirror glass unclips with a bit of care - you wind the mirror inwards as far as it will go, then insert a slightly-padded screwdriver into the gap at the outer edge of the mirror glass to help lever the plastic hooks on the back of the mirror glass off the support (that is attached to the adjuster motors). There are 4 pairs of hooks at 90 degree intervals, if you carefully get the first to disengage then the others can be persuaded step-wise. Being rough with it will snap them off. These are located about an inch inboard of the end of the glass, so you have to insert the screwdriver inwards until you can feel this, then gently lever against the mirror housing while using your fingertips to apply pressure on the mirror glass end, it should release with a bit of wiggling
although the image linked below is heated and non-original the location of the hooks is about right. With Audi original mirror glass the plastic support casing on the rear goes all the way to the end of the mirror glass in all directions rather than just being located in the middle so is actually considerably more robust than this is.