SQL> select * from a;

A                    B                    C
-------------------- -------------------- ----------
1                    1b                   1c
2                    1                    2c
3                    3                    3c

SQL> variable rtn varchar2(20);
SQL> begin
  2  update a set b = '2b' WHERE A=2 returning c into :rtn;
  3  dbms_output.put_line(:rtn);
  4  end;
  5  /
2c

PL/SQL procedure successfully completed.

------解决方案--------------------
PL/SQL Developer

declare
 rtn number;
begin  
update student set age=276 where id=11 returning age into rtn;
dbms_output.put_line(rtn);
end;