If my memory serves me correctly, I believe I had the dealership do this when some other service was done.
However, I found this:
Getting Started - Adjusting Tailgate Height
First, locate your Lexus' tailgate operating button up under the rear part of the tailgate when it is open. Pressing that button causes the door to begin to close. A second push will stop it. Use that second push feature to stop the door when it has lowered to the height that is right for you. If you are short of stature and can't reach it, we suggest asking a taller family member to help out.
Adjustment Height Is Easy
After you have pushed the button a second time and you are sure it is the height you would like it to open to in the future, simply push and hold that same button again for two full seconds. We suggest counting off "One chimpanzee, two chimpanzees", but use your own method if you so prefer. Once you have held it that long, the RX or NX is going to buzz at you. Don't take it personally. It might (should) buzz four times to let you know that it has accepted your settings. Now push it once to let it down and try it out.