They have kits that will do this, google "screw removal kit."
They basically destroy the screw, so you'll need a replacement...
Pretty much anything you do will destroy the screw, but it's stripped so it's already worthless.
If you don't want to get the kit, you can also just slot the head and take it out. If the head is too recessed it's not an option, but if you have access to it with a cutting wheel on a Dremel, you can slot it and take it out with a flathead driver.