You unscrew it, swap it out and add the gas to the tap part. There is still lots of carbonation in the beer, so you are just losing the headspace.
Be sure you unscrew the bottle standing up and slowly to release the gas that has built up already. I have had bottles over flow when opening to fast, but I think I used too much priming sugar in those.
I have had some problems screwing in the CO2 cartridges, keep that red tab that holds the cartridge holder. Ive noticed ill screw it in and it will not puncture the gas. I dont wanna over tighten, but if I slowly loosen the cartridge it will catch it as I back out. I liked the little system, purchased several more empty bottles and had fun with it. I just upgraded to a keggerator though, so im not using the tap-a-draft stuff much anymore.