<?php if ( ! defined( 'ABSPATH' ) ) exit;

global $wpdb;

$wpdb->update( "{$wpdb->usermeta}",
	array(
		'meta_value'    => serialize( array() ),
	),
	array(
		'meta_key'      => 'um_account_secure_fields',
	),
	array(
		'%s'
	),
	array(
		'%s'
	)
);