After replacing the display or with someone else's image recorded on the µSD card, you can try this method to change the vcom value.
1. Make sure that you have firmware 4.38.23171 installed on your Kobo Nia, if not, update it.
2. Now install the firmware
kobo-nia-temporary-update-4.41.23145.zip on top of 4.38.23171.
In this firmware, I simply changed the drivers so that it would install on Kobo Nia and it would be possible
change vcom value.
3. Change vcom values following these instructions:
https://www.ifixit.com/Guide/How+to+...Display/170983
4. After the change, do a hard reset using the button or turning on devmode.
Developer options - Factory reset your eReader.
5. Update to the latest firmware (currently 4.38.23171).
I tested this on Kobo Nia, but do so at your own risk.