I makes a difference; what brand of die, as in Dillon or what?
If a Dillon, there should be a small pin that holds the seating stem in place, remove and stem will come out through die mouth. If too large diameter bullet was used it may require effort/force!
If other brand die there are several ways the stem is held in place, most commonly threaded! It could be from above or from mouth of die, and there could be an adapter bushing involved. Pretty much: push everything out the die mouth, and that can be problematic if the threaded stem is very thin!
If all else fails return to die manufacturer's customer service for removal.
Ivan