https://wccftech.com/how-to-downgrade-ios-12-3-1-to-ios-12-3-tutorial/

Apple has released iOS 12.3.1 to the general public and if you’ve installed the latest build but want to downgrade to the previous iOS 12.3 build, follow the step by step instructions below. The procedure works the same on your iPhone, iPad or iPod Touch. Do downgrade to iOS 12.3 as soon as you can before Apple stops signing the firmware. Now, let’s get down to the tutorial.

If you’re looking to downgrade to iOS 12.3 for whatever reason, the procedure is fairly simple. Before we dive into it, do take note that Apple has stopped signing iOS 12.2, which means you can no longer downgrade to the build. However,  the signing window is still open for iOS 12.3, so do take hold of the opportunity before its too late.