There is a drain on the bottom of your water heater. Hopefully it is pointed away from the wall, some installers have an odd sense of humor. It will look like an outside faucet with a small handle or it will be a plastic "wheel" with threads for a hose in the middle. The plastic ones are bad about cracking, even if you don't touch it. It doesn't drain quickly, it does help to open a hot water valve to let air back into the tank otherwise it will glug a lot. And be careful.