Open Settings
Click System
Click Display
Under Brightness and color, move the Change brightness slider to the left
Use the keyboard brightness keys on your laptop
Open Action Center
Adjust the brightness tile if available
Open Quick Settings if available
Use the monitor buttons on an external display
Open Control Panel
Go to Power Options
Click Change plan settings
Adjust the display brightness
Use Windows Mobility Center
Press Win + X
Select Mobility Center
Move the Display brightness slider to the left