Why?? and how to update?

UPDATE (SELECT E.SAL,E.EMPNO,E.ENAME,D.DEPTNO,D.DNAME FROM DEPT d,emp e WHERE EMPNO=7566 AND D.DEPTNO=E.DEPTNO)S
 SET S.dname='S/W_DEVELOPER';






GIVE THIS ERROR :  "cannot modify a column which maps to a non key-preserved table"