Go to menu section you want to assign the drop down menu eg Main menu, click on it , all the menu names will appear,
then click on any one you want to assign to open it or create a new one.
When it opens under " Parent item" by default you will see "Menu Item Root" that means it will appear normal, if you want the menu to become dropdown just change this to any menu inside there eg About us. This means your new menu will be a dropdown of about us.