Apply penetrating oil and let it soak
Use the correct size six-point socket or wrench
Tap the socket onto the bolt head with a hammer
Try a bolt extractor socket
Use locking pliers if any part of the head is exposed
Heat the surrounding metal carefully
Strike an impact tool or use an impact wrench
Cut a new slot in the bolt head and use a flathead driver
Drill a pilot hole and use a screw extractor
Drill the bolt out completely if extraction fails
Replace the bolt after removal