Editing the theme

Greetings,

I'd like to make a personal edit for this theme.
Currently my site "Name" is showing up in it's default spot on top of /greenNblack/img/header_bg.png
which on the same line also has the home menu. Because my site name is long, and I want to add more menu options, the site name is appearing behind the menu options and thus not readable

I would like for my site name to show up a little bit lower on top of greenNblack/img/sub_header.png

Which file(s) would I need to edit to do this?