Goto you root install that has folders in it such as images, includes, language, modules, etc., and you will find a themes folder.

Within the themes folder you will see both:

1. themes/yourthemename/header.html and
2. themes/yourthemename/theme.php

If the first file (themes/yourthemename/header.html) does not exist then all of your header information, including the topnav bar, are within your theme.php file.