If they're cross-threaded in threaded holes, you'll have to enlarge the hole and re tap it. If you've stripped the head, try placing a piece of electrical tape over the head of the screw to fill in the gaps. As for removing a cross-threaded screw, If it's pretty badly mangled, and you know the thread is ruined spray it with WD-40 and try and get it out with some pliers.
Different people have different methods, so wait for some of the others to reply before yanking it out with brute force.