It's time to fire up your iPhones and iPads for the first iOS update of 2019. This update contains a number of bug fixes, including issues affecting the Messages app, CarPlay, and HomePod. It also contains a number of patches for an array of security vulnerabilities. Check here for release notes.
Here's how to install final iOS 12.1.3 to your iOS devices.
Step 1 Connect your iDevice to PC with USB cable and launch 3uTools.
Step 2 Navigate to Flash & JB → Easy Flash and select the firmware iOS 12.1.3. There will be a popup after you click Flash, please read it carefully. If you want to save your data, please check the box "Retain User's Data While Flashing".
Please back up your iDevice regularly just in case.
Step 3 Keep a good connection and internet while flashing, wait patiently for around 5 to 10 minutes. Your device will be upgraded to final iOS 12.1.3 after reboot.