can't find software update in settings

There is no software update on versions of macOS prior to Mojave. It would be the Update toolbar item when the Mac App Store is launched. On Mojave thru Monterey, there is System Preferences : Software Update, and on Ventura, it is System Settings : General : Software Update.

Upgrading the operating system may be restricted based on the year of your Mac hardware. See the Apple () menu : About This Mac panel. Look at what is in parenthesis on the product description line. What is it?