From December 2020, Microsoft will automatically start upgrading some systems to Windows 10 version 1909 or 2004 if users do not update their PC.
This move is coming following Microsoft's announcement terminates support for 1903 version of Windows 10 (Windows 10 Home and Windows 10 Pro).
The company issued the upgrade warning two months ago and had confirmed that it would start forcing users to upgrade even if they did not want to.
So if usesWindows 10 version 1903, you will be forced to upgrade to version 1909 later this month. If you're using Windows 1909 version 10, you'll upgrade to Windows 2004 version 10 (May 2020 Update) by spring next year.
Be aware that it is not possible to block mandatory upgrades, except and if you want to tease it register Windows or Group Policy.