I agree likely it is a leaky valve, but you shouldn't need to change the whole thing. You will need shut the water off someplace before the valve. Unless you know the hot water tank has an expansion tank, if you shut the main cold water valve off, you need to open a hot water tap. Either the inlet or outlet of a hot water heater must alwaysbe open. Then unscrew the nut between the handle and valve, and remove the valve stem. It may be dirty, or have a bad rubber washer. Clean and replace if needed. Put the nut back on and turn the hot water off everything else back on.
Note, the seat the rubber washer fits against must be smooth. If not, it must be replaced if possible, or the whole valve replaced. To replace the valve, first unscrew the nut holding the tubing going up to the tank. Then sign 2 wrenches, hold the pipe coming out of the wall While unscrewing the valve.
Questions? Post back.