add_action ( 'init', 'mobile_redirect' );
function mobile_redirect() {
    if ( is_page( 'Download' )) {
        require_once( get_template_directory() . '/mobile_detect/Mobile_Detect.php');
        if ( class_exists( 'Mobile_Detect' ) ) {
            $detect = new Mobile_Detect;
            if ( $detect->isiOS() ) {
                wp_redirect( 'https://itunes.apple.com/au/app/stashd/id714814803?mt=8'); 
                exit(0); 
            }
        }
    }
}