You want to purchase a pop off valve that is rated for the same BTUs as the water heater. Pop off valves or temperature relief valves are available at home improvement centers and plumbing supply houses.
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Turn off the circuit breaker that powers the water heater. Turn off the cold water supply valve that is toward the top of the water heater. If you have a gas hot water heater, turn off the gas supply at the shut off valve on the water heater. Allow the water to cool for an hour before beginning to replace the pop off valve.
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Flip the lever on the pop off valve to allow the water inside the tank to drain below the level of the valve. Most hot water pop off valves will have a PVC pipe attaching to them to direct the flow of water to a drain beside the water heater. If there is no drain beneath your PVC pipe, place a pan under the pipe to catch the water. Shut off the pop off valve if you need to empty the container and then continue draining.
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Disconnect the PVC drain pipe from the end of the pop off valve. Grab the pop off valve with a pipe wrench and grab the PVC pipe with another pipe wrench. Turn the PVC drain pipe counter-clockwise to disconnect the pipe from the valve.
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Grab the pop off valve at the base where it connects to the water heater tank with a pipe wrench. Turn the pop off valve counter-clockwise to remove the valve from the hot water tank.
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Wrap the threads of the new pop off valve with plumber's tape. Thread the pop off valve into the hot water tank until the valve is hand tight. Turn the valve another three-quarters of a turn with the pipe wrench or until the end of the valve is pointing toward the PVC drain pipe.
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Wrap the threads on the end of the drain pipe with a new coat of plumbers tape. Thread the PVC drain pipe into the end of the pop off valve until it is hand tight. Turn the pipe another half turn with the pipe wrench.
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Turn the cold water supply back on to the hot water heater and turn on the circuit breaker to provide power to the water heater. If you have a gas water heater, remove the access panel below the gas valve to access the pilot light. Turn the valve to the "pilot" position. Insert a lit wooden match up to the pilot light and press down on the reset control beside the gas valve. Turn the valve to the "on" position when the pilot light is lit. Place the access panel back over the water heater.