There are two ways to upgrade to iOS 12, either by itunes or Over The Air(OTA). If you are updating your iphone through OTA, you have to simply follow this path: Settings > General > Software Update. But many users have complained that they are facing screen freezing and the update is not completing. It is stuck on verifying updates and seems to never finish. Here, I am going to explain some solutions of this problem. You can try these out and see if the update completes successfully.
Lock your iphone by pressing the Power button. Unlock it again. Repeat this for about 5-10 times and see if this is resolved.
Before performing hard reset, make sure you have backed up your data.Hold the "Home" and "Sleep/Wake" button at the same time. Keep holding till the screen turns off and then release once Apple Logo appears.Once your iPhone has rebooted successfully, go to Settings > General > About and make sure that the iPhone is running on the iOS 12. If not, repeat the update process.
Make sure you have a complete backup of your device on computer and you are connected to the internet.Also, make sure there is enough space in the Launch iTunes and connect your iphone to computer. Click on Device -> Summary -> Restore iPhone .Choose the backup you created, and click Restore.
If any of the above solutions are not working, you can try an alternative way to upgrade to iOS 12: upgrade with iTunes on your computer.