From your Admin Dashboard, go to Products > Products > Website Products.
Near the top left-hand side of the page, click the "Add Category" button. Once the Category is created, you can drag the category to the desired location (ie. placing "Peonies" under "Flowers").
*Editing a Category: If there is a category you don't want showing right now, ie maybe it's summer and you don't want Christmas as a category under "occasions" simply un check that box and save.
Note that moving around categories can sometimes make the front page products disappear. If this happens, just reindex to fix (or request that Floranext reindex your site's database).