To edit the XF86Config file you'll need to use a text editor, probably the most newbie friendly will be either nano or pico, to open a file in a text editor in the command line:
For you. Same with pico:
Simply give the command to open your text editor (pico, nano, vi, joe, or whatever you'd like to use) and the path to the filename, and it will open the application with that file in it. Then edit the file to your needs, save and exit. This will all need to be done as root since you are editing a system file. Anytime you use root to edit system files, it's STRONGLY recommended you do a backup of AT LEAST the file you are editing, if not a full system backup.