How do I turn off the rotation lock

See this --> Rotate the screen on your iPad - Apple Support

If you have an older model of iPad with a physical side-switch, this is likely to be toggled “on”. Switch-off the side-switch and screen rotation should be fully restored.

The behaviour of the side-switch is configurable between locking scree rotation - or muting the iPad:

Settings > General > [Use Side Switch To:] - select either Lock Rotation or Mute

For newer models of iPad without the side-switch, running recent versions of iOS/iPadOD - to enable or disable Screen Rotation, you need to access the Control Centre. Swipe downwards from the top-right corner of the screen - then tap the Screen Rotation lock icon:

If screen rotation proves to be problem with some Apps - and not others - you need to be aware that not all Apps are coded to operate in both Portrait and Landscape screen modes. Apps that have a fixed screen orientation will always appear in the orientation determined by the App Developer.

I hope this information and insight proves to be helpful.

On devices using iOS 9, swipe up from below the screen. This should bring up Control Center. Look for the icon with the lock, surrounded by the directional circle. Tap to disable / enable. Hope this helps.