If the current B-nut is just free turning try applying downward pressure while turning. But if it has to be cut, cut the bolt head off being sure not to push the shaft into the fuse. Tie a piece of thread to the bolt shaft long enough to reach through to an opening. Work the shaft and B-nut out of the fuse while leaving the thread end to end. Now you can remove the B-nut from the shaft, screw a new B-nut onto the cut shaft and pull it back through the fuse and work it through the hole. Using pliers pull the new B-nut tight using a wedge to pull against. Screw the shaft out, use a new bolt to tighten the B-nut all the way down and remove the bolt. Run ca into the hole enough to reach through and around the new B-nut. Far easier to do than to explain.