How to I get my videos on my iPhone 6 to turn to landscape?

I've been stuck with something that makes my videos stay in portrait mode on youtube and every other video site on youtube. How do I set it to be viewed in landscape mode?

If you have the latest update from apple all you have to do is swipe up from the bottom of your screen and tap the icon that has an arrow around a smaller icon. Then it should turn screen lock off.