Your iPhone & iPad are naturally good at automatically adjusting the screen brightness based on your surroundings. sometimes however you want to do this manually. Here is how. 

Adjust Brightness through Control Centre

• To open the Control Centre swipe down from the top right of your iPhone or iPad. Should your device have a Home button, then swipe up from the bottom

• Next to the Sound button, you should see the Sun icon, indicating brightness. 

• Swipe up and down to adjust brightness

• Should you desire more control, hold down on the brightness section.

Adjust through Settings

• As an alternative option, you can adjust brightness through settings. 

• Tap on Settings (Cog icon) then tap on “Display & Brightness”

• You can adjust brightness using the slider. 

Source & Further Reading:  https://www.howtogeek.com/683796/how-to-adjust-the-screen-brightness-on-your-iphone-or-ipad/