Locate the pressure vacuum breaker, which is usually located just above the sprinkler manifold outside of the house.
Turn both of the ball valves on the vacuum breaker 90 degrees clockwise to shut off the flow of water.
Place an adjustable wrench around the bottom compression nut on the vacuum breaker. Turn the nut counterclockwise until the pipe below releases easily.
Pull the pipe out of the breaker.
Place a water hammer arrestor onto the end of the exposed pipe. Use a push-to-fit arrestor such as a Sharkbite brand. These arrestors do not require you to solder or cut the pipe in most circumstances.
Place the pipe threads on the other end of the arrestor into the base of the vacuum breaker and tighten the compression nut by turning it clockwise.
Turn on both of the ball valves on the vacuum breaker 90 degrees counterclockwise and turn the sprinkler timer to the "run" position to check the performance of the system.