I have two computers: a Windows 10 PC and a Mac mini. I have been using Office 2016 for many years on Windows. Recently, I subscribed to Office 365, so now I have two versions of OneNote on Windows. I continue to use the 2016 version because I prefer it to Office version (prefer is an understatement).
On the Mac, which I recently purchased, I first installed Office 365. I then went to look for OneNote 2016. There are many sites offering it for download (for example, the Apple app store), so I didn't think it would be a problem. However when I download, I get the Office installer. And if I run the Office installer, it doesn't do anything because the Office OneNote app is already installed.
The moral of the story seems to be:
If you have the free version, you can get the paid version, but if you have the paid version, you can't get the free version!
The only solution I can think of would be to uninstall office (or maybe just the OneNote app), then download the free version, then reinstall office. It seems risky, so I probably won't try it.