There is no section in the App Store (which I get to when clicking SW Update in the About this Mac window) that deals with OS updates. Even searching for macOS Catalina gives me compatible apps, but not an OS. I may be overlooking something, but it is quite frustrating.
Is Catalina compatible with your Mac? See: macOS Catalina is compatible with these computers - Apple Support
If it is, see instructions in How to download macOS - Apple Support
Is Catalina compatible with your Mac? See: macOS Catalina is compatible with these computers - Apple Support
If it is, see instructions in How to download macOS - Apple Support
How do I update macOS Sierra to Catalina?
