To access it you have to navigate to the Appearance / Customize/Menus.
Admin Panel → Appearance → Customize → Menus Menu Locations
This is where you set the location of a certain menu you have already created. Click Menu Locations and you will see two dropdowns, one for the header menu & one for the footer menu. In the dropdown, the items displayed are the names of the menus previously created. If you don’t have any items here, you need to create a menu.
Created Menus Below the menu locations, you can see the menus you have already created. If you click a menu you have created, you will see the following:
Menu items - these are the pages and links you have added whilst creating your menu.
Reorder items & add items - Possibility to reorder the menu & remove items.
Delete menu - Button to delete the menu in whole.
Display - Where the menu is displayed, either header or footer location
Menu options - a possibility to click the check radio button if you want to automatically add to the menu each next new page you add as a separate item in the menu itself.