2000 altima too rich

How do i adjust my fuel to run a little bit leaner? Is this a computer adjustment? And can i do this using a rental checker auto computer diagnostic tool that they offer for free with the manual? I have a 2000 2.4L. I think its running a bit too rich and i would just like to make some adjustments to see how it runs different or if my gas mileage will improve. Let me know Thanks