Steps to  upgrade the Mac OS Catalina OS X 10.15:

• Select Software Update from the System Preferences menu. Alternatively, use the App Store app and search for macOS Catalina under the Featured tab (+ 1) or on Apple's Catalina download page.
• To begin downloading the Catalina installer, click the Upgrade Now or Download option.
• The installer will start on its own and launch automatically after you download macOS 10.15 Catalina. To complete the upgrade and begin using macOS Catalina, follow the on-screen instructions. Select either the internal hard disc or an external hard drive to create an external macOS drive when requested.

Read More: What is the Catalina update for Mac OS all about? What are the features of this update?