You need to edit xorg.conf. If you go to the /etc/X11 directory you'll see different xorg.conf files numbered in descending order (1 being the most recent change in your xorg.conf, larger numbers approaching your first configuration). Just move one of the numbered files to xorg.conf.
Try this method of copying the file, making the changes you need, then moving it back so it is referenced as xorg.conf:
Here's a backup just in case something goes awry.
cp xorg.conf xorg.conf.backup
Edit the xorg.conf.1 file using nano as you wish.
cp xorg.conf.1 xorg.conf
This may give you an error if it will not let you replace xorg.conf with the file you edited. If it does allow you, restart your x-server or from the recovery console you can reboot.
mv xorg.conf.edit xorg.conf