Lift up the rubber boot, underneath is a lock nut and a post with a thread in it that your mirror screws into. If your mirror is loose then your lock nut is probably loose as well. Adjust your mirror support to put it into a position that allows you to adjust your mirror itself to give you the correct rear view. Use your 14 mm spanner and tighten up the lock nut, ensure your mirror does not move. push down your rubber boot. Done.
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