Once the water has drained, disconnect the garden hose. Place an absorbent towel beneath valve to catch any water that spills out when the valve is removed. Use a pipe wrench to turn the entire drain valve counter-clockwise and remove the drain valve.
Replace the drain valve with a matching hose bib, which can be found in the plumbing section of any hardware store. Apply teflon tape to the threads of the new valve before installing it to the water heater. Tighten the new valve by hand. Once tight, use the pipe wrench to securely tighten the valve in place and align it to its proper position. Do not overtighten the valve or you may strip the threads or damage the tank. If you are uncertain how tight to make it, use your best judgment and then check for leaks. If it leaks around the threads, and you properly applied teflon tape or pipe dope, then tighten the valve further.
Once the valve installation is complete, make sure the valve is closed and begin refilling the water heater. Turn on the water supply, while leaving open the tap you opened either. After a minute or two, inspect the valve for any signs of leaking.
Once water is flowing smoothly from the tap, go ahead and close the tap. Once the water heater has completely refilled, turn the power back on or turn the gas valve back to its normal position now.