All navigations/menu’s can be edited via Appearance → Menus from the select field on this page you can edit one of the following theme menu’s.
To edit the header navigation make sure you select the “Main Navigation” from the dropdown field:
Also make sure the “Primary Menu” checkbox at the bottom of the page is checked:
To add a new link to the navigation you can select the “View All” tab on the left side and select the page(s) you want to add:
To edit or delete a menu link from the navigation you can click the arrow symbol on the menu link to open the panel:
For more information about editing menus you can read the general WordPress documentation: https://codex.wordpress.org/WordPress_Menu_User_Guide