The Reset Export feature resets the compression of a file. You can use this feature if you chose the wrong compress option in the original export.
This one resets the export, everything else you did with the project is still there.
To reset export:
Navigate to the Export Book Page
Select Reset Export
You will now see the Export Page is back to how it looked before the export, and you will be able to compress the book to your liking
The phrase on the main project page will revert back to This project has not yet been exported.
Here is a video tutorial on How to Reset an Export.
Navigate back to Exporting a Book