Update iphone

If you cannot get the over the air update to work, you could update by connecting your phone to your computer and using iTunes to perform the update.

See the "Update your device using iTunes" section in this article.

Update the iOS on your iPhone, iPad, or iPod touch - Apple Support

that's a very confusing error message. You're getting that message because you're trying to install an update that is no longer being signed by Apple. You'll need to delete the installer file and download the current update (iOS 11.1.1).

1. Go to Settings > General > Storage & iCloud Usage.
2. Under Storage, select Manage Storage.
3. Locate the iOS software download in the list and tap on it.
4. Select Delete Update, and confirm you want to delete it.

It certainly should. My one concern is that if the OP is deleting the file and it's not actually deleting, something else may also be going on. But, definitely worth doing first.

I admit to not going back and reading the thread from the beginning when I posted the last answer. 🙂

Back up your phone. Restore it as new. Update. Restore from the back up.

Would the ‘tethered to iTunes’ update from the article that I recommended earlier not circumvent the already downloaded update?

If it does then it would be a less drastic option.