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 you are using Windows 10 version 1903, you will have to upgrade to version 1909 later this month. If you are using version 1909 of Windows 10, you will be upgrading to version 2004 of Windows 10 (May 2020 Update) by the spring of next year.
Be aware that it is not possible to block mandatory upgrades unless you want to compromise the Windows registry or group policy.