Problem with setting that turns light off

[AC Question]

The backlight on my phone won't turn off so of course it is using a lot of battery power. It just suddenly started doing this. How do I change the setting?

 

[B. Diddy]

Welcome to Android Central! Backlight? Do you mean that the screen is never turning off, or that the screen goes off but you can see that the blank screen is still illuminated?