I would use either Developer Tools in Firefox browser, or Inspect Element feature in Google Chrome.
While viewing your webpage, right click the area of interest (navigation) and try to pinpoint the ID of the element you need to change.
Look for the menu items since that’s what we want to change here.
Once you find the name of the attribute you’re trying to effect, you can then log into your WordPress site, and like Charles said, go to Appearance->Editor and on the right select style.css file.
Once you’re in your stylesheet, CONTROL + F (to find) your ID of interest, and there you can change the appearance using CSS.
It should be something like menu (to change your text), or sub-menu (for the position of the pop up menu when hovering).
Hope this helps, let me know your results.