Locate the pressure vacuum breaker. It's made out of brass and usually sits directly above the sprinkler control box outside the house.
Turn the ball valves on the breaker 90-degrees clockwise to stop the flow of water. None of your sprinklers will work after you shut off the valve, making it safe to repair the broken head.
Locate the damaged sprinkler head.
Insert a pair of pliers into the notch on the top of the sprinkler head and pull up to expose the nozzle and supply tube.
Hold the tube in one hand while unscrewing the nozzle counterclockwise with your other hand. The head will pull free from the tube once you've turned it several times.
Place a new nozzle on the supply tube and turn it clockwise until tight.
Release the supply tube and allow the tube to retract into the sprinkler body.
Turn the ball valves on the vacuum breaker 90-degrees counterclockwise to turn on the flow of water.
Turn the sprinklers on by setting the sprinkler timer to the "run" or "test" position. Check the performance of the new sprinkler head before turning off the sprinklers.