Open URL option allows you to open diagram exported as URL and update it in the editor. The opened diagram will be saved in the page’s attachments.
After you click the Open URL button, you will see the following dialog.
After you insert the URL and click the Open button, the diagram will be opened in the editor.
When you have finished working with the diagram, click the Save & Exit button to save the changes and exit editor or Exit to discard the changes made (if any) and leave.
After that, you'll see the preview of the uploaded diagram.
