The short answer is... hold the power button down and the brightness will progress through twenty levels of light intensity from 15 lumens to 800 lumens.
RA operates in one of three modes: LOW, HIGH and FLASH. Each of those modes is adjustable. With RA in LOW or HIGH mode, push and hold the button to adjust brightness. While the button is held, RA will cycle through its brightness settings. When the desired setting is reached, simply release the button. RA will remember the new setting for that mode. When the lowest or brightest setting is reached, RA will flash three times before adjusting the brightness in the opposite direction.
You can find more details in the User Manual.