Upgrade from previous versions to OS X 10.11 (El Capitan) Reinstall El Capitan on a computer already running it. Reinstall while keeping files, applications, and settings; Reinstall while erasing everything on your computer; Upgrade from previous versions to OS X 10.11 (El Capitan) From the Apple menu, select App Store. When the App Store. Climb that mountain — Apple releases OS X 10.11 El Capitan to the App Store Free download for all supported Macs. Andrew Cunningham - Sep 30, 2015 5:49 pm UTC.
I know that in the past if I've previously downloaded OS X El Capitan from the App Store I could download it again from the Purchased tab in the App Store, however now that macOS Sierra is out I'm not able to download it as it says INSTALLED (and grayed out) even on a system running OS X Mountain Lion 10.8.5. Pressing any key combination, e.g control, option, etc. doesn't change the button to DOWNLOAD. All other previous versions of OS X can still be downloaded from my Purchased tab in App Store as they still say DOWNLOAD and are not grayed out, thus active.
El Capitan Download App Store
El Capitan Mac App Store Link
The image below is from a system running OS X Mountain Lion 10.8.5, does not have OS X El Capitan installed on it or even the Install OS X El Capitan.app on its filesystem, yet it says it's installed and grayed out.