The above advice is correct, IF, there is enough of the broken stud to grab. If not the task is a little harder. Many had had luck with grinding a screwdriver slot in the end of the stud with a Dremel tool and a cutoff disc. Then you can simply unscrew it.
Others have been able to tap the broken stud around (counter clockwise) with a very sharp center punch or similar tool.
Worst case scenario, for a seized stud, or if you used red Locktite, is to drill the stud out and use a tool called an 'Easy-Out'. They also make drill bits that run in reverse to help remove broken stud/bolts.
Good luck.
Tom