I started one of my comics thinking it was going to be an all ages sort of thing.

And, well, over time it became an outlet for the more venemous aspects of my humor.

How do I change the book's rating from "kid-friendly" to "not so much"?

Go into your comic's control panel. In the first/leftmost tab, there's a few links that say 'display advanced options' or something like that... if you click on those, the page will expand, and changing the rating will be one of the things you can do. :] The thumbnail doesn't always update very quickly, but if that's bothering you, you may be able to force a change by uploading a new thumbnail.