I am trying to edit my header.php to call the navigation label of a page as opposed to the page title. Here is the existing code…
<title><?php wp_title( '' ); ?></title>
What is the code for calling or echoing the navigation label of a menu item?

Thanks.
What you need is to access the
wp_nav_menu_objects, by first rendering the menu to extract the label in order to use it on the pagetitle.You can see a working solution well explained on this links:
How to get current-menu-item title as variable?
first render the menu and then display it later