Do note that Windows Update may just not offer a major update because such as Windows 10 2004 because of concerns about driver compatibility. It happened to me because Microsoft and the manufacturer of the PC didn't come to an agreement about driver compatibility. Windows update gave me warning about updating to the new Windows 10 version (to 2004 or 20H2), but there was no update for it in Windows Update, I had to download the update manually to get it installed. Also had to do that manually for a previous version as well.
There is a bug in the Windows Update client on Windows 8.1, and it can't update itself.