If you want to free up some space on your device, you can remove downloaded showcases in the Showcase Workshop app.
To do this, find the presentation you want to remove and tap the small grey circle with a line in it directly below it.
You'll get a pop-up asking if you'd like to remove this presentation, so don't worry about accidentally removing one!
Once you remove a presentation, the grey icon will change to a green cloud. This means that you can re-download it at any time by clicking the cloud icon!