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 "Flashlight" to turn it on. Tap it again to turn it off. If you cannot find this icon, 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 Flashlight icon, drag and drop it to the top menu for easy access.