Trying to update my OS and i cannot find an option in my system preferences

only catalina and big sur and maybe mojave has the update in system preferences

older version updates from the app store under the updates tab

when it's a full version update then one does not do that but use app store and choose the version

macOS - How to Upgrade - Apple

As Learned colleague @Rudegar has has provided the link and make very sure the computer qualities for the Big Sur upGrade.