With our responsive Joomla templates, the main header menu is replaced by a mobile friendly version when viewed on smaller screen devices. This menu is opened via a toggle switch at the top right of your page. By default this toggle switch uses the text 'MENU'.
To edit this text, open your templates responsive-nav.js (../templates/[YourTemplate/js/responsive-nav/responsive-nav.js) and edit the following to your own taste (line 10)...
label: 'MENU',
Joomla 3.x allows you to edit this file from within your Joomla administration. From your template manager (Extensions -> Template Manager), click the template name under the column titled 'Template'. You will be directed to your template customiser which will allow you to edit your template files.