OK, not sure why you would do an upgrade "because you always do", especially with all the issues associated with High Sierra. And hardly any of them were fixed with the OS 10.13.1 update that came out yesterday.
Also, I asked what Mac OS were you previously using. Your answer, iMac, is not an OS, it's a Mac model. I suspect you were using Sierra, is that correct?
That's good that you made a backup before you upgraded to High Sierra. So, what you can do is the following:
1. Restart your iMac from the (hidden) Recovery HD partition. This link explains how to do that, and what you can do:
https://support.apple.com/en-us/HT201314
2. Next, select Disk Utility from the menu. You then need to 1) run First Aid to insure everything is OK with your internal drive, and 2) assuming everything is OK< have Disk Utility Erase and Format your internal drive.
3. Once that completes, select "Reinstall macOS". What will then happen is that via your internet connection, the software will connect to Apple servers, and install the "appropriate" Mac OS. I am unsure what that will be, although it should be Sierra, OS 10.12.6. (I go about this in a much different way, and I have more control over the process). A fresh, clean installation of OS 10.12.6 will then happen, although it will be slow.
4. Once completed, you can select the first choice, Restore from a Time Machine Backup. Choose the backup you made right before you upgraded, and that will restore all your Account settings, applications, etc.
When you get ready to upgrade to High Sierra, it would be best to do a clean installation, and not an upgrade "in place". Also, before you upgrade, make sure all of your third party applications are compatible with High Sierra. That might entail downloading and installing updates, or new versions (like Onyx, for example).