To access the file, click the kext with the right mouse button and select "Show Package Contents". If it's a system kext, it belongs to "root", you will not have write permission, then you can make a copy, edit and replace the original with Finder.
Some editors (like TextMate) require authentication if the user does not have permission to write, others (like the default Mac text editor) only reports that cannot write.
Another option is to run the editor with "sudo" in terminal
sudo /Applications/TextEdit.app/Contents/MacOS/TextEdit [space] [drag the file to terminal]
sudo chown -R 0:0 [space] [drag the file to terminal] sudo chmod -R 755 [space] [drag the file to terminal] sudo touch /System/Library/Extensions