Although I can reset purchased designs back to their defaults, I can’t find a way to reset my own designs (after deleting elements, or changing properties, etc.) back to the defaults.
The only workaround I have found so far is to delete the design, re-upload, and reload.
The copy function that @rpegg is describing for your own files was recently added to the interface here…it makes a copy of your file and opens it in the interface so that you can make any changes you want and still have an original loaded.