To use Night Shift mode, you have to be running iOS 9.3.
Apple is releasing the update for all iPhone and iPad owners on Monday, March 21.
Once you have iOS 9.3, go to Display & Brightness in your Settings app.
Tap Night Shift.
You can turn the feature on and off manually or schedule to turn it on automatically with the sunset.
You can adjust the color temperature of your display with the slider and see how it affects your display in real time. The darker of an environment you're in, the more warm you'll want your display to be.