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.