Apple seeded the fifth beta of an upcoming iOS 11.4.1 update. one week after seeding the fourth beta and over a month after releasing iOS 11.4, an update that introduced AirPlay 2 and Messages in iCloud. 

As usual, you are recommended to back up your iDevice before any changes.

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 11.4.1beta5 as the following picture, finally click "Flash". 

• Select the checkbox that "Retain User's Data While Flashing" if you want to reserve data.

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 beta5 and reboot once or twice during the process.