How to Change the application Username and Password from Backend

declare
v_user_name varchar2(30):=upper('USERNAME');
v_new_password varchar2(30):='PASSWORD';
v_status boolean;
begin
v_status:= fnd_user_pkg.ChangePassword (
username => v_user_name,
newpassword => v_new_password
);
if v_status =true then
dbms_output.put_line ('The password reset successfully for the User:'||v_user_name);
commit;
else
DBMS_OUTPUT.put_line ('Unable to reset password due to'||SQLCODE||' '||SUBSTR(SQLERRM, 1, 100));
rollback;
END if;
end;