How do you mod Primary Race

First I'd like to say what a great game this is. I'm trying my hand on modding. Hopeullay, I can get some help from all the great people here.

First, how do i replace one of the current races in the game (example: replace the Thalian) with a custom one i'm making. I don't want to mess up the current setting, so I was thinking of using the mod folder. I already have the custom race image and logo. How do i go about doing it?

Thanks