Apple has never officially released OS X El Capitan as an ISO file. Apple distributes its operating systems as .app installers (via the App Store) or .dmg disk images. Converting that official installer to an ISO is a manual process, typically used to create a bootable USB drive for older Macs or for use in virtual machines (like VMware or VirtualBox on Windows/Linux).
While many third-party sites offer ISO files, the safest method is to download the installer directly from Apple. Apple provides a disk image (.dmg) file for users who cannot upgrade through the App Store. os x el capitan download iso
To download the OS X El Capitan ISO file, follow these steps: Apple has never officially released OS X El