Yes, you can perform the upgrade at any time. (I'm a little surprised that Windows Update stopped offering it, but maybe it "takes the hint" after a while.) Visit the Download Windows 11 page and download and run the Installation Assistant. You can keep your apps, settings, and files.
I have a 3981 Desktop which I purchased last year. I took it with Windows 10 Pro installed. When the offer to upgrade to Windows 11 came in updates, I was not ready to take it. I would now like to do the upgrade but have a couple of questions. 1 - How do I now get the upgrade? 2 - Can the upgrade be done retaining all of my programs (apps) and files?
Upgrading to windows 11