The color change Joes, like normal joes, still have a screw in the back, even though they are sealed on the sides. Unscrewing it wont allow you to take it apart. The popular method seems to be as follows.
Heat up the figure, typically in boiling water. This will loosen the sealant on the torso sides. Once that is done, use an exacto to “pry” the sides open (start from the bottom and go up the side). Try to be as gentle as possible, as this can break the figure. Once it is open, replace the O-ring as you would any other figure. Reseal with a little bit of glue if you desire.