The problem is not the software, it’s the battery.
You can try to calibrate the battery by doing the following procedure :
* Fully charge the phone and let it charge 1 more hour
* Discharge the phone fully until it shut down
* Fully charge the phone again
If calibration works, you should get a more accurate battery level.
If calibrate the battery did not resolve the problem, your phone’s battery is failing and should be replaced.
In order to restore your phone’s battery life and get a more accurate battery level, you should try to replace the battery.