how do I unsplit my screen

You can easily manage and manipulate Split and Slide Over views.

To change a Split view to full screen, place your finger one the centre of the central divider (you’ll see a little grey drag bar) and drag the divider to the edge of the screen - then release; your split-screen view will become full-screen.

You can manipulate other screen views in a similar way. To transform a Slide Over view, to Split Screen, use the grey drag-bar at the top of the Slide Over Window; drag downwards and to towards the side of the screen. As the view changes, lift your finger.

You can also change a Slide Over to a Full Screen view. This time, using the same drag-bar, drag to the top-centre of the screen, then release.

You may find these overview Apple video guides to be helpful:

After viewing, you’ll be ready for the additional Multitasking Guide:

Use Multitasking on your iPad - Apple Support

The iPad User Guide for iPadOS is also a rich source of information. Here are links to both the downloadable and online versions of the Guide:

‎iPad User Guide on Apple Books

iPad User Guide - Apple Support

I hope this explanation and additional information is helpful in resolving any difficulties you may be having with Multitasking features of your iPad.