How do I update my Mac to ios 10.15?

Check this page for compatibility…

macOS Catalina is compatible with these computers - Apple Support

To update your Mac's operating system you can do so in System Preferences or in the window opened in About This Mac under the Apple logo.That is assuming that an update is available for your OS. Incidentally, iOS is the operating system for iPhones and for iPads, not Macs. They run a Mac OS, completely different operating systems.

click the following link to get the Catalina installer assuming your Mac is compatible: link-> How to download macOS - Apple Support