Hold the base of the pump steady and twist the barrel counter-clockwise by hand until it completely unthreads.

If you are having troubles stabilizing the base and twisting the barrel, try standing on the base and using both hands to loosen the barrel.

Lift the barrel off the base.

When removing the barrel, be sure not to lose the bump stop O-ring. It may stick to the pump base, may be stuck inside the barrel, or may fall out when you remove the barrel.