I figured this out myself.
It can be changed in the PHP code rather than in the WordPress dashboard.
Log into your web hosting (or access the files via ftp), open a file manger and find this file:
wp-content/themes/'YourTheme'/header.php
Where ‘YourTheme’ is the name of the theme you’re using. I’m using shoreditch.
Edit this file. Find this line:
<?php if ( has_nav_menu( 'primary' ) ) : ?>
Under that should be the button for the menu. The text that appears on the button should be inside the tiny little piece of php code inside the button. My one had ‘Menu’, ‘shoreditch’. Of course, all of this could be different with your theme. But this is how it worked for me.