After you’ve logged in from the Dashboard or black admin menu at the top of the page, if you are a Content Administrator, you can access the Main Menu Navigation interface:
On this screen you can add new links, including links to external websites, drag and drop links into a different order, edit links labels and URL addresses, and delete. You can also disable them so they no longer display. Check out issues around trying to add a duplicate menu item to a page within the site.
Editing in the Content Type Form
It’s also possible for you to add pages to the menus when you’re creating or editing them. At the bottom of the form check the “Provide a menu link” and fill in the fields. After the page is published it will show up in the menu.