Page Manager is a helpful feature that can be used to swap pages within a project you're working on.
Page Manager can be accessed via the button located just above the book.
From here, you'll see multiple options. In the upper left, you'll see the option to Import/Copy pages.
In the upper right, you can adjust the slider to adjust the viewing size of the pages.
In the main section, you'll see all of the pages that are in the book. From here, if you hover over a page or spread, you'll notice even more options. Clicking on the red circle icon in the upper right of a single page will remove it from the book. Clicking on the red circle icon in the bottom bar below a spread will remove the spread from the book. Clicking on the blue + icon gives you the option to add more pages.
If you click and drag a single page, you can change the location of that page within the book. If you click and drag on the bar under a spread, you can move the location of the spread within the book.
Once you're finished making changes, you can click on the button to exit Page Manager.