Officially, a free upgrade to Windows 10 is no longer possible. But with a trick, it will continue to work for free!
A special tool is needed for the free upgrade: The so-called Windows 10 Update Assistant is available for download at https://support.microsoft.com/en-us/help/3159635/windows-10-update-assistant - and in brand new version!
After startup, the handy Microsoft tool checks to see if the computer is compatible with Windows 10, and initiates the download and install the upgrade, if applicable. In the tests for this article, the free upgrade worked perfectly, but those interested should hurry: Microsoft can connect the Windows 10 Update Wizard at any time to his servers.
... or just do a Google search for "Windows 10 Update Assistant"