Windows 8.1 critically outdated

Hello,

I have recently downgraded from Windows 10 to Windows 8.1. Ran through all Windows updates and I can see no more Windows updates for my computer. However Avira Free Security Software updater keeps telling me, that I have critical vulnerability - latest version of Win 8.1 is according to it supposed to be 6.10.9600.19330. My system version in command prompt shows as 6.3.9600. I can't find version mentioned by Avira on Microsoft website and Windows Update also seems content with current state. Is it a bug in Avira or Windows Update? Can I get rid of it?

Thanks in advance for any advice.

Zdenek.