Navigate to the following folder:
C:\Users\{username}\AppData\Local\Microsoft\OneNote\16.0
You can navigate here using the search bar by replacing {username}. If navigating directly from the Windows Explorer remember that the \AppData\ folder is hidden.
To clear the cache, delete every file in the \cache\ directory. By clearing the cache all notebooks must re-sync. This effectively resets each notebook.
To delete the backups, delete every file in the \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.