Now in Application Express 3.1 installation we have two scripts to change the password of internal ADMIN account.

First one is as follows which was available before version  3.1
@UNZIP_DIR/apex/apxxepwd.sql password

Where
password is the password of the Application Express internal ADMIN account.

Second one is as follows which is new in version 3.1
@UNZIP_DIR/apex/apxchpwd.sql

When prompted enter a password for the ADMIN account.

Where:
UNZIP_DIR is the directory where you unzip your application express installation.