To edit the Navigation, go to Content > Pages > Navigation
The main site navigation is located in the website's top header and it displays the Parent (or Top Level) navigation items.
Child (or Second level) navigation will display in a drop down when the top level is hovered over.
Child navigation items can have children of their own, which are typically called Third Level navigation items. Third Level navigation Items will not show up in the main navigation but will be displayed in the sidebar of the subpages within that navigation family for the pages that are using the Subpage Default Templet.
Help Site - Navigation Help Article