to edit the config
Look in /etc/x11 to see what version of x...
type vi (name of x file)
or emacs if you like that.
if you are in insert mode in vi (hit letter i), you can change anything typed in the file like a text file.
If using vi, save changes by typing :wq
If you think you might screw things up and can't remember how to get back, first make a copy of the x file and call it something like xconfige.old or whatever. then play with the original.
Or are you trying this and it won't let you while x is running (why?)