You can change the password of a database user.
The database instance is in the ONLINE operational state.
...
1. Choose Instance ® Configuration ® Database User..
Start the next step in each case by choosing Next.
2. To change the password of a database administrator (DBA user), enter the name and password of the database system administrator (SYSDBA user).
To change the password of a database user of the STANDARD or RESOURCE class, enter the name and password of a database administrator (DBA user).
3. Select the relevant database user in the list of database users.
4. Choose Properties.
Choose Password: Change and enter the new password twice. Confirm your entries.
Note the conventions for user names and passwords.
5. Choose Close.