When you select the Open URL option while adding a 
Simple Diagrams macro, you will be able to open a diagram exported as a URL and update it in the editor. The diagram you open will be saved in the page's attachments.
After clicking the Open URL button, you will see the following dialog:
After inserting the URL and clicking the Open button, the diagram will be opened in the editor:
Once you've finished working on the diagram, click the Save & Exit button to save the changes and exit the editor, or click Exit to discard any changes and leave.
Afterward, you'll see a preview of the uploaded diagram.