Watermarks on already uploaded pictures can be changed at any time. Please keep in mind though, that changing watermarks after upload takes more time than adding it during upload. This is due to the preview pictures needing to be generated again.
If you want to apply a new watermark to uploaded pictures, simply go to the job's Photo Management area and select the new watermark from the dropdown menu there.
Should the new watermark not be displayed after 5 to 10 minutes, you may need to force a refresh of the page. Usually this is done by clicking Ctrl + F5 or Shift + F5, depending on your browser.