Phone won’t update

It is probably not related to the battery assuming that the iPhone is otherwise functioning normally. Make sure that your iPhone is setup to meet the following requirements before you attempt to update the iOS version over a Wi-Fi Network. Open Settings ➡️ General ➡️ Software Update: If there is an update available for your iPhone then it will be listed here. Follow the on-screen instructions to download and update your iPhone.

• Your iPhone must have a fast Wi-Fi connection . You cannot update over a cellular hotspot or a cellular network.
• The iPhone cannot be in Low Power mode
• The battery state of charge must be above 50%
• Your iPhone must have at least 12GB 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).

Still not resolved ? Time to make an appointment at the Apple Store or simply take the phone to an authorized Apple service shop for help.