How do I upgrade from OS Mojave to OS Catalina?

Open System Preferences and click Software Update. For more information, see How to upgrade to macOS Catalina.

First, make a full backup. It is usually when there is no backup that problems tend to arise...

Go to the App Store.

Type Catalina into the search box.

I think you can pick up from there.

Update to OS Catalina from OS Mojave 10.14.5 does not finish. No idea why. No reports.

My Mac is an iMac (Retina 5K, 27-inch, 2017)