To put the mirror back in - reverse the operation.
Why I posted this.
I hit a gate post a few days ago and the mirror broke and fell out. I couldn't get a replacement so I put the old mirror back in and it just seemed to go in very easily. When I got the new mirror I couldn't get the old broken one out - it just refused to budge. I tried levering it outwards (not sideways) from the outer edge - the reverse of what I thought I'd done to fit it but no luck. So I levered from the other edge - a bit harder and eventually I put enough sideways force on it that it came out, but it also pulled out one of the control cables and I am still trying to figure a way to repair this .... I might have to buy a new unit for £30 or so - just because I didn't understand how the glass was held in.