If the Pure is connected to the power supply, it works automatically. I can't remember with the battery, but the snooze function, for example, doesn't work with a battery.

Hallo, hier die Instruktionen aus dem Handbuch:
Display settings
Adjusting the display brightness
You can adjust the brightness of the display when Pop is switched on or in
standby. To change the brightness of Pop’s display:
1. Press the Menu button.
2. Scroll < > to ‘Display’ and press the Select button.
3. Scroll < > to ‘Backlight’ and press the Select button.
4. Scroll < > to ‘Active’ and press the Select button to change the settings that
apply when Pop is switched on, or scroll to < > ‘Standby’ and press the Select
button to change settings that apply in standby.
5. Scroll < > to ‘Level’ and press the Select button.
6. Scroll < > to choose a brightness level between ‘1’ and ‘5’.
7. Press the Select button to confirm the brightness setting.
Adjusting how long the display backlight stays on for
You can adjust how long Pop’s display backlight stays on for:
1. Press the Menu button.
2. Scroll < > to ‘Display’ and press the Select button.
3. Scroll < > to ‘Backlight’ and press the Select button.
4. Scroll < > to ‘Active’ and press the Select button to change the settings that
apply when Pop is switched on, or scroll to < > ‘Standby’ and press the Select
button to change settings that apply in standby.
5. Scroll < > to ‘Duration’ and press the Select button.
6. Scroll < > and press the Select button to choose one of the following options:
On: Choose this option if you want the backlight to always stay on.
Off: Choose this option if you want the backlight to turn off after 10 seconds

You can click through the settings using the Menu, Select and arrow buttons. There are settings for the display and the clock.
And there is a manual that can be found online.