How do I downgrade from High Sierra back to El Capitan?

There is no downgrade, it has to be a clean install on either your current HD or another drive or another partition. You will also either need:

• to currently own El Capitan AND have it in your possession in some way, shape or form.

• your computer shipped with it when purchased and it would be available from Apple using Recovery.

About macOS Recovery - Apple Support

• A computer that was eligible for El Capitan but not any OS X variants after El Capitan.

if your computer was eligible for Sierra and you did not download El Capitan you may no longer be able to get it from the Apple Store as an option. In this event only Sierra and High Sierra would be downloadable unless you have another Apple Computer that is eligible to upgrade to El Capitan and currently has an OS installed that predates it and is not eligible for Sierra, in which case El Capitan should be available to you from the Ap Store

How to download OS X El Capitan - Apple Support

Before you proceed first backup

How to use Time Machine to back up or restore your Mac - Apple Support

then create a bootable instance of El Capitan

Either with directions from Apple

Create a bootable installer for macOS - Apple Support

or use a 3rd party solution

http://diskmakerx.com/

then format the drive from Disk Utility in Recovery mode

Disk Utility for macOS Sierra: Erase a volume using Disk Utility

then install the USB bootable devices image to a new partition
(or you will need to reformat your current drive if it's your only option for an install.)

then use Migration Assistant to restore you account and 3rd party applications to your system.

Move your content to a new Mac - Apple Support

If your Purchases history still has the older version shown, you

may be lucky and see if it will allow you a re-download to use it.

In my instance, the mac being upgraded has more than one internal

hard disk drive; so I chose to partition the mostly empty 1-TB HDD,

to make two 500GB. And original OS X 10.9(.5) remains untouched.

Not all have luxury of an extra internal HDD or large one to partition.

Mine has two separate drives, both partitioned for upgrade potential.

A separate external drive suited for clones or second system installs

gives one some options when there is only one internal HDD.

Be sure to create a bootable USB installer when the option is provided.

Then you have an offline key to access this resource before it's gone.

Good luck & happy trails! 🙂

you must boot that computer using EC or previous but which does support the machine.