You will need an installer for El Capitan or an unaltered backup of El Capitan prior to installing High Sierra. Downgrading requires you erase the drive then install El Capitan. This must be done from another drive than the one you want to use. If your computer came with two software restore DVDs, then start by installing from Disk1. Otherwise, use this process:
Clean Install of El Capitan or Later on a Clean Disk
- Restart the computer. Immediately after the chime hold down the Command-Option-Rkeys until a globe appears.
- The Utility Menu will appear in from 5-20 minutes. Be patient.
- Select Disk Utility and click on the Continue button.
- When Disk Utility loads select the drive (usually, the out-dented entry) from the side list.
- Click on the Erase tab in Disk Utility's main window. A panel will drop down.
- Set the partition scheme to GUID.
- Set the Format type to Mac OS Extended (Journaled.)
- Click on the Apply button, then click on the Done button when it activates.
- Quit Disk Utility and return to the Utility Menu.
- Select Reinstall OS X and click on the Continue button.
This should install the version of OS X that was pre-installed when the computer was new.