Instructions
From Alaskan King Crab:
Cooking Frozen Crab Legs:
Most frozen crab legs are already pre-cooked so they will just need to be heated. When cooking frozen crab legs, we have found the best way is to place them in a colander or steamer over rapidly boiling water. The pot should be about one-third of the way filled with water. You will need to cover the pot and steam your frozen crab legs for about ten minutes or until they are completely heated.
Other sources I've found have been as low as 5 minutes. Really, you're just re-heating cooked legs.