Make sure that your iPhone is setup to meet the following requirements before you attempt to update the iOS version-
* Your iPhone must have a fast Wi-Fi connection; you cannot update over cellular data or a cellular hotspot
* Ensure that you do not have a beta profile installed from a previous version - If you do then remove the beta profile and restart your iPhone.
* The iPhone can not be in Low Power mode
* The battery state of charge must be above 50%
* Your iPhone must have at least 6GB or more free storage space
* Your iPhone cannot be connected through VPN
* You should leave the phone plugged in until it finishes updating, both to assure that it remains above 50% and to keep Wi-Fi enabled (Wi-Fi turns off when the phone sleeps if not connected to power).
Axel F.