Hi Drakoniano;
Using one finger, press the top of the lever towards the tank (backwards) and keep it there; this will free up the ratchet and allow the screen to rotate backwards and forwards. This lowers and raises the screen - if it is in the lowest position already it will only move forwards, which raises the top. There are four other positions, and if you release the lever you'll feel the ratchet engage and keep the screen in place as you move it. The "button" moves backwards / forwards, not sideways!
Turning the knob on the lever will raise / lower the headlamp, which is on a plate connected to this lever by a cable; this allows you to adjust for different ride heights (clockwise raises the beam) which result in carrying luggage and / or a pillion. Useful; if the dip beam goes (after about 11,000 kilometres) you can put it on the lowest position and ride without blinding people coming the other way!
Hope that this helps
Good riding,
CC