Flattened PDFs cannot be edited, as they were meant to be like that when they were flattened.
However, one could use text recognition from that very image → then paste that into a new PDF.
One could take screenshots of the pictures found in the flat PDF → paste that into a new PDF.
When it comes to forms or checkboxes, they just have to be re-created from scratch.
While that’s not un-flattening, replicating the PDF is the closest method for you to get to the original file.