Tap Wallpapers & Brightness
to adjust the brightness of the screen by dragging the slider, or turn on Auto-Brightness. When Auto-Brightness is on, the device's ambient light sensor measures the current level of lighting and uses that for future brightness adjustments.
When Auto-Brightness is enabled, you may notice that the brightness slider has moved to match the current light condition based on what was set before. If you adjust the brightness, the setting will learn to match the set brightness for that light condition.
Screen brightness affects battery life. Dim the screen to extend the time before you need to recharge iPad, or use Auto-Brightness.
You can also adjust brightness with Control Center:
You can access Control Center from anywhere in iOS—including the Lock screen. To access Control Center, swipe up from the bottom of the screen. To close Control Center, swipe down, tap the top of the screen, or press the Home button.