first step is to soak it in a penetrating oil, something like Seafoam or Liquid Wrench. https://www.youtube.com/watch?v=dObEK7V-TFU

Then you can either cut a slot in it with a dremel, use a screw extractor, or tap it out yourself using a screw tap kit (I put these in the order you should try this in.

If that doesn't work replace the whole thing.