How do you remove a rusted screw?

Depends on where and how rusted. WD-40 may help. Putting a screwdriver in the slot and hitting it once or twice with a hammer may break it loose if it is in something that will survive the hit. Heating it with a propane torch will loosen it if that is possible. Sometimes you have to drill it out and put a larger one in it's place. An impact screwdriver can be used. You beat on the end while turning the screw.