My iPad is stuck on vertical orientation. How do I change it?

If you swipe up from the bottom you should get to Control Center, where you will see your recently used apps and a set of controls: brightness, volume, camera, etc.

The one you need has an icon of a padlock in a circle with an arrow - This locks or unlocks screen rotation.

At at the moment the padlock will be locked. If you press the icon it should ‘unlock’, and you will then be able to rotate your screen when you leave Control Center. If you wish to lock your screen in its new position you need to again swipe from the bottom and click the padlock icon to lock the screen.