Use the largest manual screwdriver that fits your screw. If possible, grip the screwdriver handle with a wrench to get more leverage.
[header] How to remove a stripped screw [title] Maximize the grip strength. [step] If you can still grip the screw head with a screwdriver, try one last time to remove it by hand. Follow these instructions first to maximize your chances : [substeps] If the screw is fastened to metal, spray on penetrating oil, such as wd40, and let sit at least fifteen minutes.