There is actually…first the menu button code is found in the style.css file around line 2189. Best to do this change with a custom CSS file at least and not directly with this file. The code that does the button looks like this:
1. if you are using a child theme, you can simply retype the label you want
2. using language translation, you can override it.
The label in question is found in the header.php file around line 67 and looks like this:
The “Site Menu” is the part you are referring to. This is setup so that the theme can be translated to most if not all languages which these text strings are found and allows a person to translate/change it, usually with a program such as PoEdit.
If you are already using a child theme for your site, you can simply copy the header.php over into it and then change that “Site Menu” part.