Loosen the existing screws with the appropriate screwdriver.
Position the mirror as you desire it.
Retighten the screws on each side of the mirror with the screwdriver. One screw on each side of the mirror has to be tightened to keep the mirror from pivoting.
Remove the stripped screws from the pivot holes in the mirror's frame stand, using a screwdriver. If they just keep spinning, grip the screw head with a pair of vise-grip pliers and pull it out. You may have to twist it counterclockwise.
Wrap the threads of two new screws with plumber's tape.
Thread the new screws into the pivot holes of the mirror's frame stand. Adjust the mirror to the position you desire and then tighten down the new screws. If the screws are loose even after new ones have been screwed in, remove those and replace them with larger-diameter screws.