Here is complete guide how to downgrade iOS 13 beta to iOS 12.3.

MUST READ: If you downgrade iOS 13 to iOS 12.3, you will not be able to use the backup that was made on iOS 13 via iCloud or iTunes. You can restore data only from a backup copy of iOS 12.3. If you do not have such a backup, then you need to manually save all photos and other necessary information before rolling back to iOS 12.3.

Step 1: Download the iOS 12.3 IPSW file for your iPhone or iPad model

Step 2: Open iTunes on the Mac or Windows PC

Step 3: Connect the iPhone, iPad, or iPod touch running iOS 13 beta

Step 4: Select the iPhone or iPad in iTunes:

Step 5: Select the iOS 11.4.1 IPSW file that was downloaded

Step 6: Let iTunes restore back to iOS 12.3, the device will reboot when finished

If you installed a developer profile on your iPhone, you need to
delete this profile and update before the official release of iOS via our simple guide.

Please note that  …

– you need to make sure you update iTunes to the latest version.

– the downgrade process includes the complete removal of data. Further data recovery from a compatible backup may take a long time.

– make sure you have downloaded the correct firmware file for your device.