How to change default db2 database admin username to something else in Linux?
It is not possible to directly rename the db2 instance name, But following steps can be used to move the db to new instance


  Take the backup of DB's and provide 755 permissions to backup images. Offline backup is recommended
  db2stop command to stop the instance
  drop current instance name by using db2idrop command
  create new instance by using db2icrt command
  once instance is created, switch to the instance id and  please start the instance by db2start command
  Restore the db to new instance name by using below command incase of auto storage
   db2 restore db  from  ON  NEWLOGPATH 
  If there are user defined tablespaces, Which will need redirect restore
  db2 connect to 


or Run below command to grant DBADM access to dbroot.

db2 " grant DBADM WITH DATAACCESS on database to user dbroot";