By clicking on "Menu" in the main menu you will be able to edit the main menu. There are 3 components to the menu editing interface:
Top bar - This is where you can select what menu you want to edit. Depending on your website you may have multiple menus, for example one menu for your main navigation menu and one for your website footer or perhaps some other secondary menu.
Selectable menu items - On the left hand side you will see sections with items underneath each section. These items can be added to your main menu (which is shown on the right hand side). So if you want to add the Contact page to your menu, view the pages box, select Contact and click the Add to Menu button.
Menu preview - On the right hand side is a preview of your menu. When you add menu items to your menu they will be added to the menu preview. Once you are happy with your menu click the Save Menu button on the far right hand side.
You can re-arrange menu items by simply dragging and dropping them into position. By dragging a menu item below and just to the right of another menu item you can make the menu item a child item and it will appear as a drop down.
By clicking on the menu item in the menu preview it will expand the menu item with additional options. This includes being able to delete the menu item or rename the menu item. For example your page might be called Contact Us but in the menu you just want it to be called Contact.