There are two ways. One is described here:
https://www.mobileread.com/forums/sh...d.php?t=212162
The other is to go to the Search box on your Kobo, and, as long as you have a local search (not the Kobo store or Wikipedia) selected, type this in, and press enter:
DevModeOn
Nothing will SEEM to have happened, but when you go to the main settings menu (the one accessible from the home screen) and select Settings | Device Information | Developer Options (which would NOT have been there before you did the Search command above), you'll see an "Invert Screen" checkbox.
All best wishes.