For the DB Control Release 11.2 and higher, you need to set the environment variable ORACLE_UNQNAME to the value of the DB_UNIQUE_NAME database parameter.
Steps :
1.Stop the DB Control
On Unix
$ emctl stop dbconsole
On Windows Open a Command Window and type
**> emctl stop dbconsole
2.Check that the DB Control is stopped
On Unix$ emctl status dbconsole
On Windows
Open a Command Window and type
**>emctl status dbconsole
3.Connect to the database as a user with DBA privilege with SQL*Plus and execute:
SQL> alter user sysman identified by ; 4.Check the new password
SQL> connect sysman/[@database_alias] 5.From the database directory $ORACLE_HOME/bin, execute:
On Unix
On Unix
$ emctl setpasswd dbconsole
Provide the new SYSMAN password On Windows
**>: emctl setpasswd dbconsoleProvide the new SYSMAN password6.Restart the DB Control
On Unix
On Unix
$ emctl start dbconsole On Windows
Open a DOS Command Window and type
**>: emctl start dbconsole Thank you
Osama Mustafa 
Technology Geek 