Hello
It seems that I found a topic which covers exactly your question:
https://www.mobileread.com/forums/sho...d.php?t=220794
There is a post from Boxtor where he explain how you can adjust the VCOM voltage => with the recovery update (cause I think the chip flash methode is too risky). But I don't know if it still works.
Good luck
PS: I've got the same problem with the screen voltage, but with a M96. Unfortenately there aren't many info about the VCOM voltage in the net.
PPS: Here are the instruction how you can go into the recovery mode:
https://www.mobileread.com/forums/sho...d.php?t=242726. It was written for a T68, but I think it will work on a M92 too