You are correct that the “Open Menu” button is gone in your website. This is because the website isn’t built with Webflow’s Navbar component, and is instead built from scratch using divs and interactions.
Since this is the case, you’ll just a couple more steps than usual. This video will help show you how to access the menu to make changes. Be sure to set the display setting back to Display: None after you’re done making your changes to ensure that the website’s interactions work as intended.