Thank you, Songquan, it was a good hint! Appearance->Menus->Manage in Customizer did the job. But it is also possible to create Drop Down Lists directly in Appearance->Menus. You can simply drag the items under “Menu Structure” slightly to the right to make them children of the item above, such creating the desired hierarchy. And you can also use “Custom Link” for creating Product Categories, just putting the link to your category in the URL field, like http://www.domain.com/?product_cat=categoryname.
It worked fine, but, interestingly, not for all pages. Some pages of my site are still showing the old menu configuration, some others even the more older one. Page reload doesn’t help. Is it a problem of browser, WP or Symbiostock Theme?