Hi,
Press and hold the little button for 10-12 seconds to put the gauge into calibration mode. Select the "PPM" icon. It will then prompt you to press the button again to start callibration. Then drive precisely 2 miles and press the button again.
Job done...easy.
If you have the built-in fuel level gauge, that can also be set from the same calibration menu. Instead of selecting PPM, select "Fuel Level" (or something similar). You can then select the full and empty ranges. The only problem is that the fuel tank sender unit is apparently an optional extra although it doesn't appear on any of the options lists....yet.
I hope that helps.
Paul