1. Go to your Apps > 7. Layout > Navigation
2. Select "Edit" to the navigation you want to make edits to.
3. To add a new menu item, scroll to the very bottom and click on "Add another Nav item".
4. Fill out all appropriate fields for the new navigation item. Make sure when linking within site pages or modules to use the slug only (i.e. everything after the site url (yoursiteurl/xxx)
5. Click and drag on the double blue, directional arrows and place it where you want it in the order.
6. To make the nav item a sub menu item, click once on the right blue arrow found left of the nav item.
7. Scroll back down to the very bottom and hit "Save".