Menus can be managed under Appearance / Menus. This theme contains two static menu locations – Header Menu and Footer Menu. More info about managing your menus can be found in the WordPress Codex.
Header Mega Menu
To display the submenu of a header menu item as a mega menu, insert “lsvr-megamenu” into the CSS Classes field. This will work only for top level items and only if the menu’s location is set to Header Menu.