Open the app tray (usually an icon that looks like small squares) at the bottom of your phone and scroll until you see Settings. Once in there you will likely come across a tab or menu option for brightness or display. HTC, for instance, lists this under Display & gesture. Samsung, on the other hand, will often put the slider directly in the dropdown which is accessible by swiping down from the notification bar.
You can slide the brightness up and down and manually set things however we recommend using the auto-adjust if possible. This way you screen doesn’t get any brighter than it needs be and helps extend the life of your battery.
Android 4.2 Jelly Bean (and later) also gives users a quick and easy way to access display settings. A menu of quick settings appears on top of the home screen letting users hop directly to the same toggles. Get to your brightness by swiping down and tapping the brightness icon.