Old VS New sub-menus in Shopify: How to create both
In September 2017, Shopify introduced a new sub-menu system 100% more user-friendly than the previous one. You can now simply drag & drop menu items under other ones in order to create a sub-menu. This is currently available only for the new stores.
The NEW way
Go to Online Store > Navigation (or go to /admin/menus)
Click Add New (or go to /admin/menus/new)
Then you can add up to 3 levels of menu items