If it takes the D style coupler, the American Standard for kegs, then it's fairly easy. I'm not sure what that is though.
If it is, put the keg on it's side, take a big wrench or some other tool to push down the ball to depressurize the keg completely. After that, get a small but strong flathead screwdriver, put it in one of the notches (closest to the end of the retaining ring you see) and pry that end of the ring out, then you will be able to pull it out completely. Then turn the spear (it will only turn a bit) and the notches will line up, then you can pull the entire thing out. Clean, sanitize, siphon beer in, put spear back in, start the retaining ring, then use pliers to clamp it in. After that you can connect a coupler to it and force carb it normally. You can also naturally carbonate it, which I have done and it turned out great.