Go to the Theme Options page and clicking on the Header tab. Scroll down to the bottom and you’ll see a field for a “Mobile Navigation Label.” Enter the text you’d like to have there and save your options.
Just to double check. Change it to “Menu” and save your options. Since this is an older site that has been updated, the default “Navigation” text may not have been saved to your database yet. That’s one way it could appear in your options page but not show on your site. If it works you can always change it back to whatever you’d like it to say.