The theme navigation can be edited under Appearance → Menus by selecting the “Primary Navigation” from the select field:

Also make sure the “Primary Menu” checkbox at the bottom of the page is checked:

Adding new links

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:

Edit existing links

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