From the Notification Panel

1. Swipe from the top of the screen to the bottom to open the Notification Panel.
   
2. The display brightness adjustment slider is just below the Quick Settings Panel shortcuts.
   
3. Touch and drag the slider to the right for a brighter screen or to the left for a darker screen.

From the Display Settings

1. From the Home screen, touch Apps .
2. Touch Settings .
3. Touch Display .
4. Touch and slide the Brightness level bar to the right for a brighter screen or to the left for a darker screen.
   

Still have questions? Click here for more help from the official Samsung page.