Download Links for 10.12 Sierra, 10.11 El Capitan & 10.10 Yosemite
For 10.12.6, 10.11.6 and 10.10.5 you can download the installer directly from Apple’s servers.
10.12.6 Sierra:
How to upgrade to macOS Sierra – Apple Support
10.11.6 El Capitan:
How to upgrade to OS X El Capitan – Apple Support
10.10.5 Yosemite:
How to upgrade to OS X Yosemite – Apple Support