SELECT COMPANY
       ,[LINENO]
       ,DOCUMENTNO
       ,SELLTOCUSTOMERNO
       ,CASE 
         WHEN TYPE = 'credit' THEN DOCUMENTNO
         ELSE TYPE
       END AS TYPE
       ,NO
       ,QUANTITY
       ,AMOUNT
       ,POSTINGDATE
FROM   
  DBO.SOURCE

SELECT     [lineno], documentno, type
FROM         source
WHERE     (type = 'credit')

update destination set documenttype=? where [lineno]=?