After successfully seeding different betas of iOS 12.4, Apple finally releases the iOS 12.3.1 with bug fixes for VoLTE and Messages, bug fixes & improvements.
Downloading iOS 12.3.1
You can download the iOS 12.3.1 update with two different methods. Either from your iPhone via OTA(Over the Air) through Settings >> Software Update >> Download and Install and using iTunes.
How to Install iOS 12.3.1 Using OTA Software Update:
Go to Settings >> General >> Software Update and Hit download and Install.
If your device has already jailbroken then the steps mentioned above does not apply. So you need to install directly or manually from iTunes as follows;
How to Install iOS 12.3.1 Using iTunes
Make sure to back up your device before performing this action
Connect your device to iTunes (Latest Version) > Select your device > And hit restore if you want clean install iOS 12.3.1 or hit update if you want to move from the previous iOS version.
It will first download the .ipsw firmware file and it begins with the restore process. But while doing this you need good internet connectivity. If you lose your internet connectivity you cannot resume your download. So it’s better to restore or update your device via iTunes manually.
#1. To install iOS 12.3.1 on your device like iPhone-iPad, Simply Download the iOS 12.2 IPSW link (firmware file) for your device from http://ipsw.me
#2. After you have downloaded the iOS 12.3.1 ipsw file, Open iTunes >> Go to Summary of your iPhone >> Hit Shift key and Click on Restore iPhone or Update iPhone.
Note: Restore means clean iOS 12.3.1 update. The update means just moving from a previous iOS version to iOS 12.3.1 update with no changes in your data.
#3. Now select the downloaded iOS 12.3.1 IPSW file.
#4. The restoring process takes a while so don’t panic. After rebooting several times your device will load with the latest iOS 12.3.1 software version.