It is very simple to restore a title that is currently archived! Just go to your Manage Titles page and change the dropdown to display Archived Titles.
After clicking on the title name of the book you wish to restore, you’ll see a link for each title that says Restore Title. Just press that link and that title will move from the Archived list to the Active list.
You should then check the title record to make sure all the info is still correct. The book will remain Private until you change the Availability Settings.
If you want to also replace the content file with a new version, please see here for instructions.