8/9/2013 - numerical value: 8 divided by 9 divided by 2013.
You must use the to_date() function to convert the string to a date:
UPDATE CCO.ORDER_CREATION SET "Doc_Date" = to_date('08/09/2013', 'dd/mm/yyyy') WHERE "Document_Number" IN (3032310739,3032310740,3032310738);
You may need to adjust the format mask, as it is unclear whether you have August 9 or September 8
Alternatively, you can use the ANSI date literal (the format is always yyyy-mm-dd for the ANSI SQL date literal):
UPDATE CCO.ORDER_CREATION SET "Doc_Date" = DATE '2013-09-08' WHERE "Document_Number" IN (3032310739,3032310740,3032310738);
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