If it's anything like the mirror-hanging method used in the 626 and 323,
there's a somewhat hidden (not intentionally, just hard to spot due to
location unless you're actively looking for it) screw right near (just
above, in the case of the 626/323) the joint, on the body of the mirror.
Tightening/loosening that screw also tightens/loosens the mirror on the
mount. But be careful - Tightening it too hard will crack or strip the
plastic piece inside, turning it into junk, and leaving the mirror in
permanent "flopped" position. Loosening it too much will drop that same
piece off the screw, with the same effect, and you'll have the devil's
own time trying to rattle it back into position so you can thread the
screw back into it.

DAMHIKT - the answer will likely end up being "99 and 44/100ths percent
pure" profanity :) The lessons offered by "the school of hard knocks"
are effective, but learning them is usually NO kind of fun, if you take
my meaning...

I've got an aftermarket lighted mirror on mine, but it sometimes gets loose,
too.  It has an allen screw that tightens it up.  Look around on it and see
if yours has one.