At this point you can unhook the hose at the drain valve. You may have a little clean up to do. Turn the water back on and let the tank fill back up. When the tank is full, check for leaks around the new hot water heater pressure relief valve. Not leaking? Good Job!
For an electric heater you want to turn the breaker back on. Gas hot water heaters need to have the gas valve turned to on. Make sure the pilot is still lit and it fires up when you turn it on. After the water heats up you can double check the valve for leaks. That's it, you're done.