UPDATE a SET a.MMDWNO = '21' FROM OPENQUERY(NMIIFLIB, 'select * from MVXCDTANSN.MITMAS WHERE MMITTY = ''25''') a
UPDATE OPENQUERY(NMIIFLIB, 'select * from MVXCDTANSN.MITMAS where MMITTY = ''25''') SET MMDWNO = 21