Hi, and welcome to the Community! 
The Navigator is great for moving chapters around. 
Here’s how:
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Open the Navigator by clicking the compass-icon in the bottom-left corner of the screen.
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Click and hold down the mouse button on the chapter, and move it up and down in the Navigator.
Here’s a quick video on it:
Do you have both numbered chapters and non-numbered chapters in your book?
If that’s the case, I think you’ll find a solution here: Problems Adding Numbered Chapters
Let me know how it goes! 
