Use your finger to remove the bump-stop O-ring from below the pump's outlet cap threads.

Place a new bump stop O-ring into the pump.

The O-ring can be inserted into the other side of the pump body. As long as it is straight, the pump plunger will push it down to it's proper place.

Be sure the O-ring is not in any of the pump body's threads or it may become damaged during re-assembly.