I needed to manually set my liftgate adjustment the other day. It's easy.
1. Start by manually lifting the liftgate to the desired position you want it to stop at. It's heavy and there is a lot of resistance, but you can push it up and down manually to set the adjustment.
2. With the gate in your desired position, press and hold the liftgate close button until you hear two beeps.
3. Now close the lift gate using the close button.
4. Now open the lift gate using the button inside the cabin.
It should now open to the desired height you set it at.