Forcing system upgrade despite "Battery too low"
You could recalibrate the battery. You could try using an app or use method 1 on this AndroidPit help page:

Method 1:

Discharge your phone fully until it turns itself off.
Turn it on again and let it turn itself off.
Plug your phone into a charger and, without turning it on, let it charge until the on-screen or LED indicator says 100 percent.
Unplug your charger.
Turn your phone on. It's likely that the battery indicator won't say 100 percent, so plug the charger back in (leave your phone on) and continue charging until it says 100 percent on-screen as well.
Unplug your phone and restart it. If it doesn't say 100 percent, plug the charger back in until it says 100 percent on screen.
Repeat this cycle until it says 100 percent (or as close as you think it's going to get) when you start it up without being plugged in.
Now, let your battery discharge all the way down to 0 percent and let your phone turn off again.
Fully charge the battery one more time without interruption and you should have reset the Android system's battery percentage.


Don't do this too often, though, as it may reduce the battery's lifetime.