This is a generic answer but it should help you out with the basic premise of fixing a lose mirror.
The bolts that hold your side mirrors in place should be underneath the interior door panel. There will either be a smaller panel (just opposite of your mirror) that will either screw out or pop out with a screwdriver, or you have to remove the whole panel to get at the bolts. If you have to remove the panel be careful to not damage any power windows/door lock wires if your vehicle is equiped with those options, and be ready to head to a napa or kragen/checker or similar parts store for new panel clips if needed. Tighten the bolts down, check that it's tight and replace the panel.