How do you fix an engine with low compression in all 6 cylinders?

The repair depends on what is causing the low compression. Low compression can be caused by a blown head gasket, cracked cylinder head, burnt or damaged valves, worn or cracked piston rings, scored block cylinders. It's kind of hard to imagine that an engine would have low compression on all 6 cylinders unless something major is wrong, or the compression gauge is not measuring properly.