It would depend on how they were originally cooked. Boiled or steamed dumplings are best reheated via steaming (gentle heating) to preserve the texture of the filling, whilst deep-fried or pan-fried dumplings can be microwaved or baked in the oven. Note that the bottom of potstickers may not be crispy after re-heating- you would need to pan-fry again (after reheating) to crisp it up.