From a Home screen, navigate: Apps icon > Settings > Display & gestures.
If Settings unavailable, tap the Dropdown menu then select Alphabetical. If still unavailable, refer to Manage App Icons.
Tap Brightness level.
Adjust the brightness level as desired by sliding the green dot right or left then tap OK.
Setting can't be manually adjusted if Automatic brightness is enabled (enabled when a check mark is present).
To adjust automatically, tap (check) Automatic brightness.