RCBS (and others) have a stuck case remover set you can buy. But there is no need to spend the $ and inconvenience of buying one.
1. Back off (unscrew) the expander plug/decapping rod.
2. Drill out the primer pocket using a 3/16" dill bit. This can be done in a drill press, or hold the die in a benchvise and use a drill motor. Best yet, use a lathe and 3-jaw chuck.
3. Thread the hole using a 1/4-20 tap.
4. Using an appropriate size ( 1/2") mechanics socket, washer, and 1/4-20 bolt or cap screw, wrench the screw in until the stuck case is extracted from the sizing die.
The extractor kits contains the drill, tap, screw, washer, and a collar that fits against the die.