How to modify mobile nav menu text in theme

You can leverage the use of the gettext filter to change the text. Add this code to your child theme’s functions.php or to a plugin:

add_filter('gettext', 'wpse248225_change_text', 20, 3 );
function wpse248225_change_text( $translated_text, $untranslated_text, $domain ) {
    if ( NECTAR_THEME_NAME !== $domain ) {
        return $translated_text;        
    }

    // Make the changes to the text
    switch( $untranslated_text ) {
            // Change 'Menu' text
            case 'Menu':
                $translated_text = __( 'something different', 'text_domain' );
            break;

            // add more items
     }

    return $translated_text;        
}