To view a live stream or video in full screen on a desktop browser, click on the icon on the bottom right of the player. The player will switch to full screen. To exit full screen, click the icon again or press [esc] on your keyboard.
If you are watching a video via your mobile device (iOS or Android) tap the same icon in the bottom right of the player.
If you try rotating and notice no change, make sure your Portrait Orientation Lock setting is turned off.