iOS 12.1.2 beta update appears to mostly focus on under-the-hood changes. Release notes for the update don’t mention any major UI changes or new features, it mostly contains just bug fixes and improvements.
Now you can upgrade to iOS 12.1.2 beta using 3uTools. Don't forget to Cutomized back up your important data in case of data loss.
Step 1 Connect your iOS device to PC with genuine USB cable and launch 3uTools.
Step 2 Navigate to Flash & JB → Easy Flash, selet firmware iOS 12.1.2beta. If you want to save data, check the option "Retain User's Data While Flashing". And then click Flash.
Step 3 Keep a good connection and internet while flashing, wait patiently for around 5 to 10 minutes. Your device will be upgraded to iOS 12.1.2 beta after reboot.