Turn the temperature dial on your hot water heater all the way down. Turn off the gas control knob and wait at least 10 minutes to allow any gas to clear away.
Turn the gas control knob to the "Pilot" setting. Strike a flame on a long match and hold it to the pilot. Push in the reset button located by the gas control knob and hold it in for one full minute.
Release the reset button. The button should pop back out. Turn on the gas control knob and reset the temperature to your preference.