Drop-down menu options will appears when you select an item with a mouse. The item you select is generally at the top of the display screen, and the menu appears just below it,
Open the menu settings by clicking on Menu (see the above video)
Click on settings.
Click on Arrange (Next to Type and settings Link)
Hove over the Menu Links and few arrow will appears.
Use the Arrow button to create Drop-down List.
Click on the Publish button to make the changes.