If you want to remove the old water pressure relief valve on the water heater, there are some very simple steps that you can take. However, before you take off the valve, you should ask yourself if this is a safe thing to do. Water pressure relief valves are designed to take pressure off of the water heater, and if this is not done, you risk a major explosion. Removing a valve to replace it is reasonable, but a leaking valve usually indicates that pressure is too high.
Step 1 - Turn off the Water
Before you begin, make sure that your water heater is turned off, and then switch off the main water supply pipe to your home. Make sure that any wires and electrical connections are removed, either by lifting them out of the way, or taking them down temporarily.
Step 2 - Remove the Valve
Check that the valve is not pinned to the water heater by a brace. If it is, remove this using a screwdriver. Then take your wrench, and place it over the water pressure relief valve. Turn it slowly anti-clockwise until it comes away from the water heater.