macOS Catalina - Technical Specifications - Apple Support (IN)
Always use Time Machine or an external drive to back up your Mac before a major OS transition. how to update macos high sierra 10.13.6 to 10.15
Because High Sierra is an older version, the update might not appear automatically in your "Software Update" menu. Method 1: The Mac App Store (Recommended) macOS Catalina - Technical Specifications - Apple Support