Learn About the macOS Downgrade Process
If you don't like macOS High Sierra you can downgrade to macOS Sierra, but it's going to take some time. We recommend learning about this process before you upgrade, just so you know that you are prepared to do it if you need to.
It's best if you have a good backup of your Mac on macOS Sierra, since that will simplify going back to the older version of macOS. To do this, you basically need to erase your Mac and restore from a Time Machine Backup made on macOS Sierra.
Spend a minute making sure you have a good backup and looking over the downgrade process so that you know what you may be getting into.