You can't improve this picture, as it is over processed, is full of artefacts, has washed out colours and missing detail and all a lot of compression artefacts:


In addition, your picture is highly underexposed as shown by the histogram:

You also would need to edit out these identifiers (IP issue):

You didn't include the EXIF data, but my guess is that you used a small sensor camera and in camera processing. This is fine for your holiday snapshots or a newspaper snapshot, but not for stock, where you need to have clean pictures for all uses.