I have a leaking valve on my waterheater , do I need to replace pressure valve , if so what is correct way of doing so ?

What valve is leaking?

Keep it simple, He's already said it's the T & P valve that's leaking.
Mikey, Do you have a sprinkler system installed or a backflow preventer out next to the meter? If so the T & P valve's OK and you need to install a expansion tank on the cold water supply to the heater. Regards, Tom