I would follow the previous advise
Though if you insist on downgrading, which I too would not do.
This method will WIPE ALL Data and there is No DATA Recovery - Period.
To perform this action will require booting from a Bootable Installer.
Use the Specific Instructions for El Capitan
How to download and install macOS - Apple Support
The Bootable Installer can Only be performed on an Apple Computer
This will have to be performed from a Qualifying Computer to run the version of macOS to be made on the Bootable Installer.
Notation: If the computer being used to perform this action is Too New or Too Old to qualify to run the version of macOS - this computer can’t be used.
Alternative is, to gain access to a Qualifying Apple Computer from a family member, friend or associate.
Shutdown computer and disconnect all external drive Except the newly created Bootable Installer.
Restart and immediately hold the OPTION key until the Startup Manager appears and choose the USB Drive.
It will present options >> Disk Utilities >> View >> View ALL attached Drives.
Choose the Upper Most Drive ( not the volumes indented and list below ).
Formatting for macOS 10.13 High Sierra and below requires HFS Journaled with the GUID Partition Map
Once that is done >> backup out of Disk Utilities and choose install macOS.
Follow the prompts and it may automatically reboot several time.
Upon a final reboot - Setup Assist will present with the newer version of macOS.
