MERGE INTO ttt a
USING
(
SELECT *
  FROM (SELECT cd
             , sdt
             , edt
             , TO_CHAR(TO_DATE(
               LEAD(sdt) OVER(PARTITION BY cd ORDER By sdt)
               , 'yyyy-mm-dd') - 1, 'yyyy-mm-dd') edt_new
          FROM ttt
        )
 WHERE edt != edt_new
) b
ON (a.cd  = b.cd
AND a.sdt = b.sdt)
WHEN MATCHED THEN
UPDATE
   SET edt = b.edt_new
;