As your design progresses you sometimes need to add additional storeys "midway" up a building (or anywhere for that matter.)
To do this select the Storey where you want to insert your new storey and click the Insert Stroey button in the Edit Toolbar.
The new Storey will be inserted above the currently highlighted storey.
To add additional storey's, just click the button again (and again.)
Just below the Insert Storey button is a Delete Storey button that allows you to just as easily remove stories.