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Via System Settings
Go to the System Settings
There are several ways to access the system settings:
From the Ubuntu taskbar
By typing system settings in the Unity dash search box
By clicking the wheel in the top-right corner of the screen and selecting System Settings
Select Brightness and Lock
Move the slider to the left to decrease the brightness level
Note: If you don’t like to go to the System Settings to adjust the brightness every time, or if your laptop doesn’t come with a dedicated brightness control button, you can install the indicator-brightness utility to directly adjust the screen brightness from your desktop.
To install it:
Open the terminal by pressing Ctrl + Alt + T
In the terminal window type the following lines (one by one, i.e. once you enter the first line, press Enter, and repeat the same procedure for the other lines):