Update problem

If you have an iPhone 7, you cannot update to iOS 12.3.1, you have to update to iOS 12.4, the latest version. You will not get it to update to iOS 12.3.1. You might as well stop it. If you were doing it over the air, see this for how to delete the update and download the correct one. Get help with over-the-air iOS updates - Apple Support

And it will stay right there unless you read the link I provided you to stop it, delete it, and then try to install the correct version. You need to be installing iOS 12.4.

If you are using the standard update procedure (Update your iPhone, iPad, or iPod touch - Apple Support ) there's no way to update your iphone to 12.3.X. Your unit will be updated to the latest iOS version (now 12.4).

Regards

Giulio