By selecting the Appearance > Menus option from the WordPress admin sidebar, you can set the top navigation by creating a menu, and on the left under Theme Locations set the Main Menu dropdown to be your newly created menu and then click save. This will allocate your created menu to the header navigation of the theme.
The theme supports two levels of drop-down, so you can manage your menu items and allow for deeper navigation. This theme also have two menu in header. You just need to check the box of "Theme Locations" at the menu settings.