From your dashboard, go to Products > Products > Website Products.

Near the top left-hand side of the page, click the "Add Category" button. Make sure the category is enabled and set to display in the nav bar.

Once the Category is created, you can drag the category to the desired location (ie. placing "Peonies" under "Flowers").

If there is a category you don't want showing right now (eg. it's January and you want to hide the Christmas category under Occasions), simply un-check that box. The change is automatically saved.

Note that moving categories around can sometimes hide the front page products. If this happens, contact Floranext support to reindex your site.