Key to accessing and controlling multi-tasking views is the small “three-dot” button that is found at the top-centre edge of each App Window. Tap the multitasking button at the top of the slide-over window to expose a menu - from which you can change the window view.
While you can't directly swap a split-view from left to right and vice versa, you can instead switch one of the two Apps to full screen - and then, re-select a split screen view, choosing a right or left split as you prefer.
More information about using multi-tasking can be found in this support page:
Use multitasking on your iPad - Apple Support
The iPad User Guide is also a rich source of information about using iPad and its many features. The Guide is available online using a web browser (such as Safari):
iPad User Guide - Apple Support