It is a steel stud threaded into an aluminum head. If you can't salvage the thread with a die like 16VAlex has suggested, I would soak the base of the stud with a penetration fluid or WD-40. I would then grip the stud below the threads (if possible) with a small vise-grip. The vise grip has to be really clamped tight and have some room to swing. There are also tools to remove studs. I would then go to the dealer or parts store and get a replacement. The replacement can be tightened down using two nuts tightened against each other seperated by a lock washer.