It is clamped in by a spring tensioned clamp. You need to pull it down to get it loose and this can be a bit difficult sometimes. You haven't got a rear view mirror with electronic parts, so you cannot break it.
Try to pull it down, it should work!

greetingz,

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1990 300SL-24
1993 C250D with a minor 600+k kilometer
www.MBenzNL.com(the Netherlands)