How do I adjust the headlamp levelling based on vehicle load?

Headlamp levelling is adjusted using a switch with positions 0 through 3 depending on vehicle load. Position 0 is for driver only or driver and front passenger; position 1 is for all seats occupied with no boot load; position 2 is for all seats occupied plus an evenly distributed load in the boot; and position 3 is for driver only plus an evenly distributed load in the boot. (Page 50)