Proceed through the guided installation screens.
Apple hosts the OS X El Capitan installer on their content delivery network servers. Mac Os X El Capitan 10.11.6 Installer Dmg Download
macOS 10.11 El Capitan - End of Life - SCS Computing Facilities Proceed through the guided installation screens
Disclaimer: Ensure you back up all data using Time Machine before attempting a clean installation or OS downgrade. Mac Os X El Capitan 10.11.6 Installer Dmg Download
❌ Does support 2016+ Macs (they require macOS Sierra or newer).