Welcome to the Forums.
Are you referring to the battery-charge level at which power-saving mode starts, causing a dimmer screen image? If so, it is in Settings, General, Battery, Battery Saver.
If you mean screen brightness, it is in Settings, Display, Brightness. Brightness can also be adjusted by using two fingers to pull down from the top of the screen, revealing a brightness slider control, among other things.
If you mean the length of time that the screen stays on when you stop using the phone, it is in Settings, General, Accessibility, Screen Timeout.
Posted via Android Central App