To change the DbaaS_Monitor password of an DBCS instance:
Login as oracle (via ssh opc@) then
cd /u01/app/oracle/product/ords/
and
java -jar ords.war user dbaas_monitor "OSAdmin" Enter a password for user dbaas_monitor: Confirm password for user dbaas_monitor: May 17, 2018 9:52:26 PM oracle.dbtools.standalone.ModifyUser execute INFO: Created user: dbaas_monitor in file: /u01/app/oracle/product/ords/conf/ords/credentials