There are lots of long blogs about how to do this. Some are written for the beta version of High Sierra and are wrong. Here's the quick and dirty on trashing your APFS disk in High Sierra and go back to HFS.
APFS is brand new disk partition type that Apple introduced in High Sierra. Although it has some benefits, I don't care for it. Anything that apple releases that is brand new, be weary, there are usually issues with it. Also, if you're a mac tech reading this, upgrading OSes or converting from HFS to APFS to solve performance problems is not a solution. If you need help understanding why, email me.
***WARNING: THIS WILL DELETE THE CONTENTS OF YOUR HARD DRIVE. MAKE A BACKUP***
That's it. You can now re-install, re-store, re-image to your HFS disk however you like. You're welcome.
We're some guys and gals in San Francisco who fix macs. We can remote in and help you with stuff. It's like our job or whatever.
(five one zero) 761-6559