How to download macOS Sierra
Apple recommends High Sierra, the latest version of macOS, to get the latest security and compatibility enhancements. But if you need the previous version, you can download Sierra here:
https://support.apple.com/en-us/HT208202

How to download OS X El Capitan
OS X El Capitan remains available for Mac computers that can't upgrade to macOS High Sierra, or that need to upgrade to El Capitan first:
https://support.apple.com/en-us/HT206886