Even after completely downloading the iOS 12 (iOS 12.2/12.3) update, chances are that your iPhone can simply get stuck on the iOS 12 update verifying prompt. This issue can arise due to a corrupt or incomplete software download, a problem with your Apple ID, or any other software-related issue. There are also times the problem can be fixed automatically.
Restart the update: The best way to fix this problem is by restarting the update. Firstly, turn off your device by pressing the Power button. Afterward, turn it on again and go to its Settings > General > Software Update to restart the updating process.
Reset Apple ID: Reset your Apple ID to fix the verifying error related to your account. Just go to your phone settings and tap on your Apple ID. Sign out from it, wait for a while, and sign back in to resolve the issue.
Force restart iDevice: If you still face the same error, then try to forcefully restart your device. This would break its present power cycle and might fix the verification error. To do this, press the Power + Home/Volume Down button on your device at the same time for at least 10 seconds.
Reset all settings: If there is a problem with your phone’s settings, then you can choose to reset them as well. To do this, go to its Settings > General > Reset and tap on “Reset All Settings”. Confirm your choice by providing the passcode of your device. Your phone would be restarted with the default settings. Afterward, try to do the iOS 12 (iOS 12.2/12.3) update once again.