It really is not that complicated:
On your computer download
this file (right click on this link and choose Save as or Download or whatever ; also post-download check that browser didn't stupidly add ".txt" as the file extension [the filename should be "device.lua" not "device.lua.txt", if it is incorrect rename it to delete the ".txt" at the end] then copy it and paste & overwrite that over existing file in "koreader/frontend/device/kobo
/" folder
on your Kobo that is connected via USB cable.