Editing main horizontal navigation bar
-If it is in the Default for the page (Probably is), open:
Dashboard > Pages and Themes > Page Types (On the top of the section).
-If it is in the Theme itself, you have to Edit the theme. Just find the /themes/(theme name) folder, and check the php files that have the menu. As you find it, just edit it.
If it is HTML, PHP, JS or what, we can only be sure looking at the files themselves. Probably HTML, no one does this much PHP to render something that breaks this easily, though it can be done.
If you need any more "technical" help, just post where you got with this info. It's probably quite easy. :D
Edit: Also, check the navigation as Pritam said. Don't stop with the quick-fix, it will still be "breakable", it's best to have something that works. A bit more work now, but makes life easier in the future.
Quickest/dirtiest fix may be adding a beneficial link/page name by creating new jpeg sized to replace the original(links, contact, map, whatever) and either creating a new page to match the menu or even linking to one of the other pages temporarily.
Sorta halfway done, but will buy you time while considering if you want to replace the entire nav section
CSS nowadays does a job even prettier than the images on the menu, anyways.