You just unthread them, and install a new one.
Often, if they are leaking, you will need to consider "why" they are leaking. Is the spring old, or is there something on the seat? Is the pressure too high? and if so why? Is the water too hot? Does the thermostat work right?
: I have the same problem as the poster below. Is there a "how to" document on changing the T&P valve?
: : : Hi Terry,
: : : I just noticed that the water valve at the top of my 30 gal. gas water heater is leaking. Is it supposed to do this? It has a tag attached that warns against replacing it yourself and talks about water pressure. The valve seems to leak more when I'm using the hot water(dishwasher etc.)should I get a repairman or is there something I can do. There is a tray under the heater I assume to catch water if it leaks, but I've really never noticed it leaking before. We've had this water for five years. It has never had anything done to it. Any thing you can tell me would be appreciated.
: : : Thanks,
: : : Myrtis Kail
: : Myrtis,
: : Sounds like the T&P valve. Temperature and Pressure.
: : This thing is to allow excess presure to be released so the tank doesn't become a bomb.
: : If you have a bit of muscle, you should be able to replace it.
: : Keep in mind, that five years is not a long time for it to start leaking on you though. If you have a check-valve water meter, or a pressure reducer on the system, you may want to check into an expansion tank while you are at it to allow for the expansion of the hot water. Terry