The ALTER PASSWORD statement ( alter_password_statement) is required to alter the password of a database user.
<alter_password_statement>::= ALTER PASSWORD <old_password> TO <new_password> | ALTER PASSWORD <user_name> <new_password> <old_password>::= <password> <new_password>::= <password>
SQL Tutorial, Database Users and Their Privileges
You can change the password for the current database user using ALTER PASSWORD <old_password> TO <new_password>. The old password must be the same as the password stored in the database catalog for the current user.
If you want to change a password for a database user using user_name, then the current database user must be the database system administrator.
The new password must be specified in the CONNECT statement when the user next opens a database session.
Database Administration, Conventions for User Names and Passwords