If an update is available, you'll see it listed here. Click Update Now or Upgrade Now to begin the process. Depending on your internet connection, this might take a while.
Because you are running High Sierra, you may not see Catalina in "Software Update" within System Preferences. You must use the App Store: macOS - How to Upgrade - Apple (SI) how to update macos high sierra 10136 to 1015
macOS High Sierra (10.13) was a solid, reliable operating system, but it is now outdated, unsupported, and missing critical security updates. Upgrading to macOS Catalina (10.15) offers a more modern interface, better security, and access to newer apps. However, this is a significant jump that requires preparation. If an update is available, you'll see it listed here
Since you are on High Sierra, the update may not appear automatically in "Software Update." You should use the method: Updating and Upgrading macOS High sierra - Apple Community Because you are running High Sierra, you may