To adjust the output power, just click the wattage adjustment button. The wattage increases by 1W per click.
You can only increase the wattage. To reduce the wattage, you need to set the wattage to its maximum value and then click the fire button again to set the value to 1W. Once you set the value to 1W, you can proceed to increase the wattage again by clicking the wattage adjustment button.