To change split screen to full screen on iPad, follow these steps:
Locate the app you want to use in full screen mode.
Swipe up from the bottom of the screen to access the Dock.
Tap and hold the app you want to use in full screen mode.
Drag the app to the left or right side of the screen until it fills the entire screen.
Release your finger from the screen.
Alternatively, you can tap the green icon in the upper left corner of the app to switch to full screen mode.
If you want to disable split screen completely, you can do so in the settings. Go to Settings > Home Screen & Dock > Multitasking and toggle off "Allow Multiple Apps." This will disable split screen and Slide Over mode.