Apple has released the first iOS 11.4.1 beta to registered developers for testing on iPhone, iPad, and iPod touch. iOS 11.4.1 likely focuses on bug fixes to any issues created by yesterday’s iOS 11.4 release.
You're suggested to back up your iDevice first, you can restore it whenever you need
Step 1: Connect your iDevice to PC using the USB cable and launch 3uTools on your PC.
Step 2: Click "Flash & JB" -> "Easy Flash", then select the firmware iOS 11.4.1 beta as the following picture, finally click "Flash".
If you want to save iDevice's data, you need to select the checkbox that "Retain User's Data While 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 11.4.1 beta 1 freely.