how to change display from horizontal to vertical

Open the Control Center by swiping from top to bottom in the top right corner of your iPad, then you need to make sure the Rotation Lock button is off, it shouldn't be colored blue in order to be off.

Rotate the screen on your iPad - Apple Support

If your screen still doesn't rotate, try checking to see if your iPad has a side switch:

Check to if your iPad has a side switch

To switch between Portrait and Landscape view, you generally only need to rotate the iPad to the desired screen orientation.

For newer models of iPad without a side-switch, you may need to disable your Screen Rotation Lock. To do so, 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.

(*) For compatible Apps that are intended for iPhone, iPadOS15 will now automatically rotate the App to display in Landscape screen orientation. There is no manual configuration or setting to enable this feature.

However, Apps designed for iPad that are expressly coded to force Portrait screen orientation, iPadOS15 will honour the intended Portrait mode. iPadOS15 will not override the App coding.

Do you mean that it is stuck in landscape mode and you wanted to go back to portrait mode? Have you tried restarting the iPad? That would be the first thing I would try. Also, I would want to make sure that the lock rotation is not enabled