In addition to the release of the betas of iOS 12.4, Apple is also working on the iOS 12.3. The iOS 12.3.1 was released at last weekends, aiming to fix a few issues, for all iOS 12 compatible devices.

You may follow the instruction below to get the update.

Step 1 Connect your iDevice to PC with an original USB cable and launch 3uTools. 

Step 2 Go to Flash & JB →Easy Flash, select firmware iOS 12.4beta2. If you want to upgrade with data, please check the box "Retain User's Data While Flashing". There will be a pop reminder after you click "Flash", please read it with caution.

You are recommended to back up your iDevice regularly in case of data loss.

Step 3 Keep the stabilization of network and USB connection while flashing, wait patiently for around 5 to 10 minutes. Your iDevice will be restored to iOS 12.4 beta 2 after reboot.