without seeing a url as a sample, there can be a few options.
If the source of the "MENU" is an image, replace the image with one that is editted as you desire. upload to the same directory as current image and the change should take effect.
If it's text, and hard-coded, you'll have to change this in the template file that contains the text. Not too hard, but need to see the site to determine where it is likely located.
It it's text, drawn from a language file, as with some plugins, this can be changed in the language file used for the menu.
My guess it's either the first or second situation unless you have a multilingual site.