You need to be running as root in order to manually edit xorg.conf. As long as you are familiar with manually editing a file, just open it up using your favorite editor (vim, emacs, pico, whatever) and start making the changes. Note: I would strongly encourage you to make a copy of the existing file, just in case you need to fall back to it.
If you have not manually edited a file, or are unsure how to go about doing it, it may be simpler/easier to just rerun the xorg configuration process by running: xorgconfig
That will take you through a series of questions asking about your monitor, your video card, etc, and the result will be a new xorg.conf file. Again, make a copy of the existing file just in case. Good luck with it and welcome to LQ -- J.W.