If you enable Include Template Comments in Page Source, then do a View Source in your browser, you'll see HTML comments in the source of your webpage that reveal what template files are responsible for creating what part of the page. (Teach a man to fish... )
The code you're after though is in public_html/layout/nouveau/header.thtml:
If you make changes, make sure to copy public_html/layout/nouveau/header.thtml to public_html/layout/nouveau/custom/header.thtml, and make your changes there. That way, they won't be overridden on an upgrade!