If you are currently running an OS older than El Capitan, you can find the update via the under the "Purchases" or "Updates" tab, provided you have previously "purchased" (downloaded for free) it with your Apple ID. If not, the hidden support link mentioned above is the primary method.
Once the download is complete, follow these steps to upgrade to El Capitan: el capitan download 10.11
Downloading the installer is only the first step. If you’re installing on a blank drive or multiple Macs, create a bootable USB: If you are currently running an OS older