Unlike minor updates, the installer for macOS Catalina is not always visible in the "Updates" tab of the Mac App Store on High Sierra machines. The most reliable method is using Apple’s official hidden support pages.
macOS Catalina entirely. Only 64-bit apps will run. how to update macos high sierra 10.13.6 to 10.15
There are two primary methods to perform the upgrade. Unlike minor updates, the installer for macOS Catalina
If you encounter an error during the installation, don't panic. Unlike minor updates
Open Disk Utility (found in Applications > Utilities) and run First Aid on your startup disk to ensure there are no errors before the OS rewrite begins. 3. How to Update: Step-by-Step