add_action('cred_submit_complete', 'my_success_action',10,2);

function my_success_action($post_id, $form_data){

//put whatever control in place to act only on given forms (if cred form ID == etc etc)

//get all child posts to the post we created/edited, this is how attachments are bound to their "parent" or owner posts
	$args = array(
		'post_parent' => $post_id,
		'post_type' => 'attachment',
	);

	$attachments = get_children( $args );

//If there are attachments​, get their ID and update the current post meta _thumbnail_id with it
  	if ( $attachments ) {
		foreach ( $attachments as $attachment ) {
			update_post_meta($post_id, '_thumbnail_id', $attachment->ID);
                }
	}

}