Apple today released iOS 11.0.3, the third official update to the new iOS 11 operating system. iOS 11.0.3 comes a little over one week after Apple released iOS 11.0.2, an update that fixed an issue causing crackling sounds to come from the iPhone 8 earpiece, and almost one month after the official release of iOS 11. Check here to know what's new in iOS 11.0.3.
As always, back up your iDevice on 3uTools before you flash it.
Step1: 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 11.0.3 as the following picture, finally click "Flash". (If you want to save your data, please select the option "Retain User's Data While Flashing")
Step3: Keep a good connection between iDevice and PC, and then just wait until this process is finished.