Once a document has been published in the public setting on Issuu, it cannot be changed to Unlisted, as the content's details are already public on the web, and have been indexed by various search engines.
To resolve this, you can instead:
1. Download the content that has been made public and delete it from your Issuu account.
2. Change the file name of the document you downloaded (the file name becomes part of the document link), then upload it as Unlisted content. This will give the content a different document link and ID.
NOTE: The stats and impressions from the old documents (the ones that were public) will not carry over to these new documents and will be removed.
Once deleted, people who have already seen the public versions will no longer be able to access that content.