Apple today released 2019's free upgrade to macOS 10.15, aka "Catalina."Available from the Mac App Store, where the upgrade was tagged at nearly 5GB or as an even larger direct download - up to 8GB from its predecessor - Catalina is free and can be installed on a wide range of Macs.
Apple posted a full list of the compatible hardware here.[ Related: Apple ships macOS Catalina; here's what you get ]Mac owners currently running 2018's Mojave (10.14) can upgrade by choosing Software Update from the System Preferences window.
However, machines running High Sierra (10.11), Sierra (10.12) or High Sierra (10.13) must upgrade from the Mac App Store.
Those few customers still relying on the ancient Lion (10.7) or Mountain Lion (10.8), must first upgrade to El Capitan before upgrading from there to Catalina.
(Good luck.)