I think you mix up APEX and database users.
Reset APEX admin user password, use apxchpwd.sql script from APEX install files
Thanks for your reply. I think apxchpwd.sql is used to change the password for APEX workspace admin. I want to know how to set up the username and password for APEX superadmin (internal admin).
I am DBA of oracle instance and am APEX workspace administrator. However, i don't know how to set up username and password of internal admin.