On a downloaded showcase, you can see on the thumbnail a small grey circle with a line in it. Like this!
Tapping this prompts a pop up asking if you'd like to delete this showcase. So it's pretty hard to accidentally delete showcases!
Deleting a downloaded showcase will place it back into your list of undownloaded showcases, which you are free to download again by tapping the green cloud icon.
Note: this will not delete the actual showcase from your device, it will just remove it from your downloaded showcases. Thus, you cannot view it offline without downloading it again when connected to internet first. Sometimes you may just want to free up some space on your device, which is why we have this feature set up the way it is.