Apple is today releasing iOS 11.4.1, watchOS 4.3.2, and tvOS 11.4.1 to the public. iOS 11.4.1 comes more than a month after the release of iOS 11.4, a major update that introduced support for Messages in iCloud and AirPlay 2.
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 11.4.1 as the following picture, finally click "Flash".
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 11.4.1 and reboot once or twice during the process.