how to change the default date format in oracle
You generally don't want to rely on implicit conversions of strings to dates.  That only leads to pain and suffering since different users will have different date formats.  Either use ANSI date literals or use the to_date function with an explicit format mask

UPDATE bb_product
   SET salestart = date '2012-06-01',
       saleend   = date '2012-06-15',
       saleprice = 8
 WHERE idProduct = 6


or

UPDATE bb_product
   SET salestart = to_date('01-JUN-2012', 'DD-MON-YYYY'),
       saleend   = to_date('15-JUN-2012', 'DD-MON-YYYY'),
       saleprice = 8
 WHERE idProduct = 6