Situatie
First, click the Quick Settings button in your taskbar, which is a hidden button located where you see your Wi-Fi, Speaker, and Battery icons in the far-right corner.
Solutie
When the Quick Settings menu pops up, locate the brightness slider, which has a sun icon beside it. Click (or tap) and drag the circle on the brightness slider to increase or decrease the brightness of your built-in display.
In a nice touch, the sun icon grows larger or smaller depending on how bright you set the slider. When you have the slider how you like it, you can close the Quick Settings menu by clicking anywhere outside the menu or selecting the Quick Settings button in the taskbar again.