The Software update on Sierra shows updates available for apps installed ion your Mac, or for your current version of macOS, but will not show updates t new macOS versions.
As I said, download the macOS installer as shown in the link provided above( it has links to macOS installers in the Mac App Store), to update your Mac to a new version of macOS. Either Ventura, or some other in between macOS version. High Sierra, Mojave, Catalina or Big Sur.
Do note, past Mojave, 32 but Apps are no longer supported and will no longer work. You will need to find 64 bit updates or replacements for your current apps.
click here ➜ 32-bit app compatibility with macOS High Sierra 10.13.4 and later - Apple Support