can't get the broken radio antenna out

I recently bought my car in great condition, other than the radio antenna is broken. It seems to have been cracked on the screw that the antenna mast threads onto, leaving about a thread and a half. I want to replace it, or even just to mount a new replacement mast to it, but I can't figure out how to remove the assembly or even just the screw to replace it. How do I do it? Thank you