Apple has come up with the latest OS upgrade iOS 12.1 with many new features. But sometimes the up gradation cannot be done in some phones sometime you will not reach the server or cannot verify the update, or the update cannot be completed it may be due to lack of storage space in your device. If the updating is taking too long it would be due to slow internet, try using WIFI and also try not to use any other apps which uses the data. Even then if the iPhone update is not coming then try update the device with iTunes, next try restarting the device.
Even after doing all this if you cannot update the iOS do not hesitate to visit iPhone service center and give us an opportunity to sort out the issue for you. Our experts are here to help you sort the issues with all your Apple products.