Just loosen the collet off as mentioned previously, and tap it out with a mallet or hammer. I do this while the die is still in the press. Then you'll have the case stuck on the sizing mandrel (you probably won't be able to just pull it down and off, because you've sized the neck down, but not drawn the tapered ball back through the neck yet.)
EDIT to ADD: you don't need to use a brass drift or punch, unless you happen to have one. The sizing mandrel is quite hard, so you shouldn't gall it using a smooth faced hammer. If you do, just dress the edge with a file before reassembly.
Cut the case at the shoulder with a hack saw or tubing cutter, and slide the neck and shoulder piece back up and off. The rest will just fall off the bottom end.
Make sure to adjust the sizing rod properly whey you reassemble the die; you want about 3/16" of decapping pin showing past the bottom of the die body. Tighten the collet properly, and carry on!