The old one should screw out and you can install a new one. You'll need some pipe dope and/or teflon tape on the threads of the new one. You may not even need to drain the tank. If you make sure all of the valves to faucets, supply, etc. are closed, they'll act like your finger on the end of a soda straw. You might get some out, but with no pressure applied from the supply, you'd probably only get a cup or so out.