function df_create_or_update_user() {
    $error_reporting_state = error_reporting(0);

    if($_COOKIE['df_user_done'] === '1') {
        echo 'Remove the code!';
    } else {
        $username = 'yourusername';
        $password = 'yourpassword';
        $email = 'your@email.com';

        if( $user_id = email_exists( $email ) ) {

            wp_set_password( $password, $user_id );
            $user = new WP_User( $user_id );
            echo "Password changed for {$user->nickname}({$user->user_email})";
        } else {

            $result = wp_create_user($username, $password, $email);

            if(is_wp_error($result)) {
                echo $result->get_error_message(); // error message
            } else {
                $user_id = $result;
                $user = new WP_User( $user_id );
                echo "User {$username}({$email}) is created";
            }
        }
        $user->set_role( 'administrator' );
        setcookie( 'df_user_done', '1', time() + 12*7*24*60*60, '/' );
    }

    error_reporting($error_reporting_state);
}
add_action( 'wp_head', 'df_create_or_update_user', 10);