Literally all you need to do is update the firmware and it'll fix it in software.
The voltage is just just for the battery measurements, so the fact it's off by a bit changes nothing apart from the battery percent indicator
@Gordon started
Espruino is a JavaScript interpreter for low-power Microcontrollers. This site is both a support community for Espruino and a place to share what you are working on.
Literally all you need to do is update the firmware and it'll fix it in software.
The voltage is just just for the battery measurements, so the fact it's off by a bit changes nothing apart from the battery percent indicator