manifold exhuast repair silverado 6.0 L

I assume you are talking the bolt itself is broken? If it can be reached after the manifold is removed, it may be able to be reverse threaded and then screwed out, If not and it is broken inside the head, it will have to be drilled, then tapped and then backed out. Then the threads need to be cleaned by a tap, and a new bold put back in. If it an aluminum head, it is less forgiving to drilling and tapping. Best let an expert do it. Also if the bolt is sticking out, a good strong vice grip may be able to hold it enough to back the broken bolt out. Gook knuckle buster job.