Open any app that uses the keyboard
Tap and hold the keyboard icon or globe icon
Select Split from the keyboard options if available
If Split is not available, go to Settings
Tap General
Tap Keyboard
Turn off One Handed Keyboard if enabled
Rotate the iPhone to landscape mode if needed
Use two thumbs to type on the split keyboard
To merge it back, tap and hold the keyboard icon or globe icon again
Select Dock and Merge or Merge and Dock if available