Change Twitter icon

Now that Twitter has changed its icon, I want to update our website, but it’s been so long since I set it up that I don’t remember how to access the social media bar at the top of the page. I don’t have a menu defined for it, and can’t find any information about this in the Flat Responsive documentation. (We are using the classic editor rather than the block one.)

The page I need help with: [log in to see the link]