How to fix iPhone Battery Percentage Getting Stuck or not Updating on iPhone 6S or iPhone 6S Plus
1. Make sure that your iPhone’s data and Wi-Fi are on and connected.
2. Open Settings, General, Date & Time.
3. Toggle the switch for Set Automatically Off and On again. The switch will turn gray, then green.
4. Restart the iPhone by holding down the Sleep/Wake and Home button until you see the Apple logo.