To adjust how high your handset lifter lifts, hold the P1 and P2 buttons at the same time. When the light flashes red and blue, let go and the lifter will raise.
Press P1 to make the lifter go higher, and P2 to make it go lower.
Once you've chosen the appropriate height, save your settings by holding down P1 and P2 once again until the lifter disengages.