Even though Apple doesn’t list this officially, you can downgrade from one version of iOS 11 to another version for a limited time
Browsing: iOS 11.4
The iOS 11.4 update is a large milestone upgrade for iPhone, iPad, and iPod touch and it could take you 10 minutes, or longer, to get it on board your device.
iOS 11.4 is the company’s fourth milestone iOS 11 update and the upgrade will likely serve as the bridge between iOS 11 and Apple’s iOS 12 update.
The iOS 11.4 update is free for the iPhone, iPad and iPod touch, but only works on a select group of devices.
iOS 11.4 update fixes iPhone 8 display issues and includes security updates as well as all the new iOS 11.3 features.