Menu
Go to Admin -> Appearance -> Menus.
All menus are managed on the same screen. Monst uses one display location, Main Navigation, for the header menu. The other menus are shown by widgets in the footer.

The list also holds the Vietnamese copies of the demo menus, shown when a visitor switches to Vietnamese (see Translate).
To change a menu, click Edit on its row. Add pages, categories, tags or custom links from the boxes on the left, drag the items to reorder them or to nest them as submenus, then save.
Main menu
The main navigation of the header. The display location is Main Navigation (box Menu settings at the bottom of the menu editor). In the demo, Home has the 5 landing pages as submenus, followed by About Us, Services, Company (Team, Portfolio, Pricing, Testimonials and more), Blog (category layouts and two sample posts) and Contact. A menu item with children opens a drop-down.

Go to Admin -> Appearance -> Menus and edit "Main menu".

Footer menus
The link columns of the footer are Custom Menu widgets. Each widget shows one menu, for example About or Quick links.
Go to Admin -> Appearance -> Menus and edit "Quick links".

To change which menu a column shows, or its title, open Admin -> Appearance -> Widgets. See Footer and Widgets.
Menus in another language
Each menu can be translated. When the site has more than one language, open the menu and use the Languages box on the right to switch to the other language. See Translate.