Apple App Store Download links for 10.15, 10.14, 10.13, 10.12, 10.11 & 10.10
The links below will bring you to an Apple Support Document that will explain how to upgrade and download macOS.
10.15.1 Catalina
How to upgrade to macOS Catalina – Apple Support
10.14.1 Mojave
How to upgrade to macOS Mojave – Apple Support
10.13.2 High Sierra
How to upgrade to macOS High Sierra – Apple Support
10.12.1 Sierra
How to upgrade to macOS Sierra – Apple Support
10.11.2 El Capitan
How to upgrade to OS X El Capitan – Apple Support