A. Stop and switch off all systems
B. Spray/throw cool water on the radiator to bring down the temperature
C. Don’t remove the radiator cap immediately. Using a thick cloth un-screw, the radiator cap slowly and without removing it allow the steam to ooze out slowly. Keep on rotating the cap till it is safe to remove it.
D. Replenish the water with the engine started, & check temperature levels
E. All of the above