Very carefully
It is a tough ring and will not break, however it is loaded like a spring and, when released, will forecefully fly away. If you should lose it, you are really screwed, so be careful.
I used a small, thin, flat screwdriver and a thin, strong pointed metal pick or probe. The pointed pick was held against one end of the ring to keep it from moving, and used the edge of the screwdriver to carefully, yet somewhat forcefully, pry the other end of the ring up and away from the groove.
Once you pry one end up, and it is resting on the side of the shaft, stop. Get a sheet or blanket, cover yourself and the pump, then continue prying it off. When it does release itself, it will not fly outside of the little tent you made, and you won't have to go on a long, frustrating hunt for it -- as I had to do with mine.
