Access the "Quick Settings" menu swiping the screen from top to bottom when the screen is on and unlocked. If you only see one row of icons, swipe down again to view more. Tap on the "Battery Saver" icon to turn it on. If you cannot find it, try scrolling to the right for more icons.
Note: if it's not in this Quick Settings menu, tap on the pencil icon. Once you've identified the Battery Saver icon, drag and drop it to the top menu for easy access.