I recently upgraded my father's computer for free. Just Googled it and download an installer, all went fine and smooth
https://support.microsoft.com/en-us/help/3159635/windows-10-update-assistant
Take a look here at this article, saying that it's still free. from April 2019
https://www.zdnet.com/article/heres-how-you-can-still-get-a-free-windows-10-upgrade/
Microsoft's much-hyped free upgrade offer for Windows 10 ended in 2016, right? Not exactly. The GWX tool may be gone, but all the other upgrade tools still work. The end result is an apparently valid digital license, and there's no evidence that the free upgrades will end any time soon.