Downgrading is not a simple process and reverting depends on your backups. If you have a SSD drive and it was converted to APFS, the drive has to be erased and reformatted as HFS+. If you have a HHD (rotational) drive, the process is easier.
Revert to previous version:
The Recovery HD partition will re-install the most current version of OS X that is currently installed on the Mac, whereas Internet Recover will re-install the version of OS X that originally came with the Mac.
Newer Mac Models can go directly to Internet Recovery by using Command+Option+R or Shift-Option-Command-R boot shortcut.
https://support.apple.com/en-us/HT201314
macOS Sierra: Revert to a previous macOS version
https://support.apple.com/kb/PH25593
If your Mac is using OS X Lion v10.7.3 or later, you can also use this method to start up from your Time Machine backup disk. Startup Manager identifies your Time Machine backup as ”EFI Boot.”
Hold down the Option key immediately after turning on or restarting your Mac.
Release the Option key when you see the Startup Manager window.
Choose “EFI Boot” as the startup disk.
If you have a Time Machine backup see instructions here:
How to Downgrade macOS High Sierra
If you don't have a Time Machine backup see instructions here:
How to Re-Install OS X with Internet Recovery on a Mac
How to downgrade from macOS High Sierra back to macOS Sierra
https://www.imore.com/how-downgrade-macos