Apple today seeded the fifth beta of an upcoming iOS 11.3 update to developers and public for testing purposes. You could update in 3uTools directly without losing data.
Step 1 Connect your iDevice to PC with USB cable and then launch 3uTools.
Step 2 After 3uTools successfully connects your iDevice, navigate to "Flash & JB" -> "Easy Flash", there are compatible iOS versions options and please select iOS 11.3 beta5 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 Please do not disconnect your iDevice until the update has finished. It can take a few minutes. Your device will be updated to iOS 11.3 beta 5 and reboot once or twice during the process.