Stop Oracle Enterprise Manager

emctl stop dbconsole
emctl status dbconsole

Connect to database as sysdba and change system password

sqlplus / as sysdba

alter user sys identified by NewPassword ;
alter user system identified by NewPassword ;
alter user dbsnmp identified by NewPassword ;
alter user sysman identified by NewPassword ;

and put your newpassword

restart your enterprise maybe you will not need to the below steps .

cd $ORACLE_HOME/sysman/config/emoms.properties :

oracle.sysman.eml.mntr.emdRepUser=SYSMAN
oracle.sysman.eml.mntr.emdRepPwd=d0355495a68cd5ae
oracle.sysman.eml.mntr.emdRepPwdEncrypted=TRUE

TO

oracle.sysman.eml.mntr.emdRepUser=SYSMAN
oracle.sysman.eml.mntr.emdRepPwd=Newpassword
oracle.sysman.eml.mntr.emdRepPwdEncrypted=FALSE

“in some database version you will not see all the parameters change what you have”

Thank you all .