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To clear the cache, delete every file in the C:\Users\cache\ directorylaytonH\AppData\Local\Microsoft\OneNote\16.0\cache directory. By clearing the cache all notebooks must re-sync. This effectively resets each notebook.
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To delete the backups, delete every file in the C:\Users\Backup\ directorylaytonH\AppData\Local\Microsoft\OneNote\16.0\Backup directory. This will free up disk space. In some cases by quite a lot (>100Gb). Note that a version history of each notebook is still kept in the cloud.