MERGE INTO BFPCYFILES.CUMST A
 USING (SELECT * FROM libjuarui.CUMST_b ) B
 ON A.CUSCUN = B.CUSCUN
WHEN MATCHED
THEN
  UPDATE
  SET
  A.CUSNA1 = B.CUSNA1,
  A.CUSSHN = B.CUSSHN,
  A.CUSLN1 = B.CUSLN1,
  A.CUSFNA = B.CUSFNA,
  A.CUSLN2 = B.CUSLN2