Upgrade to IOS 10- If you have iPhone 6 or newer, iPad Air or newer.
Upgrade to iOS 11- If you have iPhone 7 Or Newer, iPad Air 2 Or newer. If you don't mind HUGE battery drain then might as well upgrade your iPhone 6/6S to iOS 11 but expect battery to drain worst than before. Only iPhone 7 and newer has some HVVC encoding, to adopt the hardware somehow while 6/6s lacks it.