That top menu is wired up to change depending on a visitor's language preference (which is a good thing), and is dependent on the individual theme. The usual link (in the site.master page of the theme you're using) is like so:
<li><a href="<%=Utils.AbsoluteWebRoot %>contact.aspx"><%=Resources.labels.contact %></a></li>
If you changed the link to this:
<li><a href="<%=Utils.AbsoluteWebRoot %>contact.aspx">Contact Webmaster</a></li>
you would accomplish changing the displayed text to "Contact Webmaster", but it would say that for every visitor, even those who do not understand English.
If you wanted to change it to "Contact Webmaster" for English speakers but but to still allow it to translate "Contact" to other languages, you could edit the Value column in the Contact row of the labels.resx file that you'll find in
the App_Global Resources folder. Changing that field to say "Contact Webmaster" would cause the menu to display "Contact Webmaster" in English, but still translate "Contact" to other languages.