First you need to be root to edit the file - either login as root, or use the "su" command to change temporarily into root.
Then use your favorite editor (vi, pico, joe, emacs) to open wpa_supplicant.conf and make the changes you need.
What to edit depends on how your access point is set up. For WPA2, my config looks like this:
psk="<my secret key>"
Note that you may also consider using wicd to control your wireless network connection since it automagically controls your wpa_supplicant and the likes and is configured through a gtk gui.