If it is just the valve, you can replace the valve. If it is leaking around where the valve goes into the tank, you probably need a new tank. If you are referring to the T&P valve at the top of the tank, that valve may be bad, or you have other problems. The T&P valve opens for several reasons: it's worn out, the water is too hot, the pressure is too high. If it is leaking, you need to find out which is the problem (or problems). Over pressure can be caused by more than one thing, and the solution depends on what's going on.