For some reason, my Windows 10 Home has magically upgraded itself to Pro, only of course I do not have the product key for Pro so it will not activate. (I think this is because I was part of the Insider programme and then left – presumably the system did not revert to Home as it should have.)

How can I “downgrade” to Home so the system will correctly reactivate itself?

Unfortunately, clean install is your only option, you can’t downgrade from Pro to Home. Changing the key will not work. You could try the following work around then test your luck:

1. Open Registry Editor(WIN + R, type regedit, hit Enter)
2. Browse to key HKEY_Local Machine > Software > Microsoft > Windows NT > CurrentVersion
3. Change EditionID to Home (double click EditionID, change value, click OK). In your case it should at the moment show Pro
4. Change ProductName to Windows 10 Home. In your case it should at the moment show Windows 10 Professional
5. Browse to key HKEY_Local Machine > Software > Wow6432Node > Microsoft > Windows NT > CurrentVersion, change the same two values as in steps 3 and 4 above
6. Close the Registry Editor
7. Insert the Windows 10 Homeinstallation media, do an in-place upgrade by launching the setup from desktop (not booting with Windows 10 Home install media)
8. When asked, enter your validWindows 10 Home product key and select what to keep(settings, personal files and apps, only personal files or nothing)
9. Windows 10 Home in-place upgrade will be done now.