I have Windows 8.1 installed in a virtual machine, and I want to upgrade to Windows 10, but the upgrade process doesn't start. Through Windows Update, I've installed kb3035583, which is the program that allows users to upgrade to Windows 10 (the one that shows up in system tray, that has a white windows logo, and can download win 10 and upgrade later, or can download win 10 now and upgrade after it finishes downloading). But the program that allows users to upgrade to windows 10, doesn't download windows 10. I've clicked on the button that says "download and upgrade later", I've waited more than 8 hours for it to start downloading, but the downloading process does not start.
Solved. I did the upgrade from 8.1 to 10, using a Windows 10 iso image. After the upgrade completed, Windows 10 was already activated (because 8.1 was activated before upgrading to 10). It seems the 8.1 license was upgraded to a 10 license.
I think that in order to update directly via downloading the updates, we need a very fast connection.