How do I change the order of the front matter and the appendices?

From version 1.9.20, on the book's page, press "Reorder" above the chapter list. The list shows three groups: the front matter (with the contents page), the chapters, and the back matter. Drag a row by its grip or use the arrows to move it within its group. Acknowledgments and the Author's Note have a button to move them to the other side of the chapters. Press "Apply new order". The new order is used in every export and on the contents page. The same order is also shown, and can be changed, in the Export card's "Include in book and order" list.