How long has it been on 90% it maybe you are not using the stock charger or it is not providing enough amps. Does the phone work normally and decreases to 0% and can increase back to 90%. If so I would suggest you to recalibrate battery. If you still have warranty I would suggest go to them first before you Root your device.
Follow below troubleshooting steps to fix charging issues on your android device.
1st Way: Check battery charge problem or not
2nd Way: Check power adapter & cable loose port
3rd Way: Check using USB cable
4th Way: Use safe mode on your device
Safe mode not allowed third party apps of your phone. It will helpful to find which app may be causing the error.
Press & hold the power button > Touch & hold the power off> Reboot to safe mode > OK
To exit safe mode on android device, Hold the power button about 30 seconds and restart it.
5th Way: Factory data reset android phone
Settings > Backup & reset > Factory data reset > Reset phone > Erase Everything