Reset Apple Setup Assistant on High Sierra (and previous OS's, too).

The MacOS setup assistant will only run once. If you've created a user and you're unhappy with it, or you want to migrate another user over you might want to reset the OS so the SetupAssistant runs again on next boot. The setup assistant differs from the Migration Assistant by allowing you to choose a specific restore point from a time machine, rather than just the last backup. This is great if you need to go back to a stable OS before you had problems. Here's how:

• Launch Terminal from the Utilities folder
• copy in: sudo rm /var/db/.AppleSetupDone
• Enter your password.
• Restart your mac.

If you lost your admin access, you can do this process through the Safe Boot (thanks commenter*):
1. Boot into Single user mode (cmd+S)
run the following commands:
mount -uw /
rm /var/db/.AppleSetupDone
reboot
On next reboot the system will start the Setup Assistant again. You're welcome. ​