Undo the gas block screws after you have disassembled the stock and action. The stock bushing should drop out of the gas block, it also holds the gas pipe in. Replace with a new size and line up the gas pipe to the bushing slot, attach back to barrel.
Remember the battle of Athens, TN!
"Work as if you were to live 100 Years, Pray as if you were to die To-morrow." --Benjamin Franklin