Video: How to create a "dropdown" submenu
Click the "Settings" button in the left toolbar. A list of your subpages is displayed. You can drag and drop any of your subpages to arrange them. Click the "Save Changes" button when you're done.
On a website, the "parent" subpage is by default a normal page with its own content. You can change this in the main settings of your website: Settings > General Settings > Submenu Options - it can be a non-clickable item or a subpage with a list of "child" pages below it.