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.
Why? 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***
boot into the recovery partition by holding command-R during boot.
click utilities in the top menu.
type and hit enter: diskutil list
you'll see the list of all the drives. look for "APFS" then the disk identifier to the right, something like "disk0"
type then hit enter: diskutil apfs deleteContainer disk-identifier-here
That's it. You can now re-install, re-store, re-image to your HFS disk however you like. You're welcome.