Since you are running vB 4.2.0 you will have to search for those in Languages & Phrases by entering, tab_ in the Search for Text box and then tick Phrase Variable Name Only in the group, Search in ... you would be looking for something similar to this, vb_navigation_tab_tab_ode1_920_text
Both of these ways I am assuming you are doing a translation to a different language, and have multiple languages on your site. If you just want to rename then into your language, and don't have multiple languages, you can rename any tab to whatever you want in the Navigation Manager.