Grab a rubber band, and gently stretch it over the screwhead. Using an appropriate screwdriver (going up a size can help), apply steady pressure and turn. The rubber band helps by both filling in the missing parts of the screwhead and providing a nonslip surface for the driver to grab on to.
Easy Way to Extract a Stripped Screw
Too much torque can strip a fastener clean in seconds, but this fix is just as quick
