Once you have noted the difference between what your hygro reads and what it should, pull the hygro out of the bag and carefully turn the adjustment knob clockwise to increase the reading, or counterclockwise to reduce the reading.
In both cases, 1 click=1%. The display will NOT change while you are doing this. Don't worry, your changes are being made, you just have to press the small "Set" button for the hygro to accept the change.