Short answer: if your operating system is updated, you'll have the latest compatible version.
Longer answer: for the current version of macOS (Catalina), the latest Safari is included with the operating system.
For the two prior versions of macOS (Mojave, High Sierra), the latest Safari is a separate update that can be found in Software Update in System Preferences. If you do not see any available updates then your computer has the latest compatible version of Safari.
For iOS, Safari is always included in the operating system. There is no standalone update for iOS Safari regardless of whether you are running the current iOS (13) or an earlier version (e.g., iOS 12.4.1).