Hold the belly ring with one hand on the top ball and one on the bottom. On most reverse belly button rings, the decoration on the belly ring is at the top, and unlike a regular belly ring which opens by undoing the top ball, with the reverse belly rings it's the opposite.
Hold the top ball tightly and unscrew the bottom ball with your opposite hand. Most belly rings open when you turn them in a counterclockwise direction, so screw the ball toward the left to get it off.
Remove the ball and pull the belly ring gently out by pulling it up and out of the piercing hole. Voila! Your belly ring out in seconds, and now you can replace the ball on the belly ring and put another one of your stylin' belly rings in your piercing or leave your belly bare.
It's best to put the ball back on your belly ring after taking it out, so it doesn't get lost.
Keep your belly rings in a safe place where they won't get dirty or damaged. Even with proper safekeeping though, it's a good idea to sanitize your belly ring before putting it back in.