Two weeks after the original developer beta, Apple has released iOS 11.4 beta 2 for testing. iOS 11.4 includes unreleased features like AirPlay 2 and Messages on iCloud. ForNow, iOS 11.4 beta 2 is available on 3uTools. 

Always backup your iDevice using 3uTools before you do any changes. Check what's new in iOS 11.4 Beta 2 here.

Process

Step 1 Connect your iDevice to PC with USB cable and then launch 3uTools. 

Step 2 After 3uTools successfully connects your iDevice, navigate to "Flash & JB" -> "Easy Flash", there are compatible iOS versions options and please select iOS 11.4beta2 firmware. (Select the checkbox "Retain User's Data While Flashing" if you want to reserve data on your device.)

After you click "Flash", there will be a popup, read it carefully and click "OK" to start flashing.

Step 3 Please do not disconnect your iDevice until the update has finished. It can take a few minutes. Your device will be updated to iOS 11.4 beta 2 and reboot once or twice during the process.