Firstly, If you're planning on making a skin, then you should use a template like this
instead of the base skin. It's much easier. (The readme.txt contains loads of useful info as well.
As for your question, you change them in a file called PLEDIT.TXT. See the readme.txt in the above template for more info.
////Link fixed. I accidentally linked to the wrong skin before. Now it's linked to "Skinner's Atlas", the template.