Removing, easy. Just take it out. Repairing the damage, not so easy. If
you're very - make that VERY - lucky, you can thread the new plug in
correctly and it will hold. More likely, it won't. There's a product called
Helicoil that looks like a steel spring with a diamond cross-section. You
tap out the old hole with a special tap, screw in the Helicoil, break off
its driving section (all of this will make more sense when you actually see
one) and you have a new thread.
   It's one of those jobs that you should try a few times on something that
doesn't matter. Reaming out the old hole and starting the threading tap
absolutely square is difficult. If you end up off-square, your plug won't
seat properly. On the other hand, it's a pretty quick job, and your local
mechanic should be able to do it cheaply enough.

Pete (fP)