How to change xorg.conf file with permission?
Open a Terminal and then :
Open with Gedit...
gksudo gedit /etc/X11/xorg.conf
open with nano...
sudo nano /etc/X11/xorg.conf
now you can edit ..
P.D make a backup before change the file so you can revert your modification...
sudo cp /etc/X11/xorg.conf /etc/X11/xorg.conf_backup
Probably you're trying to edit the file without permissions. First make a backup of your file in case something goes wrong.
sudo cp /etc/X11/xorg.conf{,.bk}
Then, you can edit the file with you text editor, for example
gksudo gedit /etc/X11/xorg.conf
And save it.