How to use Low Power Mode on iOS9
There are two ways you can turn on Low Power Mode in iOS 9. Simply hold your home button and tell Siri, Turn Low Power Mode on. Siri will turn this on for you and show a toggle on-screen. Simply say, Turn off Low Power Mode. and Siri will turn it off for you.
The second way to do this is to go to Settings, where you can control the Low Power Mode manually. There are no options or individual settings to choose and turn on or off, just one switch.
Go to Settings, Battery, Low Power Mode, and tap On.