What's new iOS 11.4 Beta here.

It’s only been a few days since the last beta, and now Apple has released iOS 11.4 beta 6 for testing. With six betas having been released, we are nearing the end of the beta testing period. Here you could upgrade to iOS 11.4 beta 6 in 3uTools.

How to install iOS 11.4 Beta 6 using 3uTools on PC

Step 1 Connect your iOS device to PC and launch 3uTools. Remember to make sure you have a recent 3uTools backup. 

Step 2  Navigate to "Flash & JB" -> "Easy Flash". If your device is connected to 3uTools successfully, there will be compatible iOS versions options and then select iOS 11.4beta6 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 deliberately and click "OK" to start flashing.

Step 3 Keep a good connection between iDevice and PC, and then just wait patiently till this process is finished. The whole flashing process needs 5~10 minutes in general, when it's finished, then you can experience iOS 11.4 beta 6.