Windows 10 updates

Years back, when Windows 10 was being offered as a free upgrade, I downloaded the file. Since Windows 11 is now being offered as a free upgrade, I installed Windows 10, while keeping Win8.1 as my active system. I ran Windows PC Health Checkup to find out what hardware I’d need to upgrade to, but it told me that I need build 1803 or higher to run that program. Windows Update claims my system is up to date. How do I get Update to recognize that my system is NOT up to date and get it to update my system properly?