APEX Change Instance Admin password:

1- Login to DB as sysdba
2- Go under APEX installation path
3- Change to your local container in my case orclpdb
  alter session set container=orclpdb;
4- Run and provide the new password

SQL>  alter session set container=orclpdb;
SQL> @apxchpwd.sql
...set_appun.sql
================================================================================
This script can be used to change the password of an Application Express
instance administrator. If the user does not yet exist, a user record will be
created.
================================================================================
Enter the administrator's username [ADMIN]
User "ADMIN" exists.
Enter ADMIN's email [ADMIN]
Enter ADMIN's password []
Changed password of instance administrator ADMIN.