functions.php

<?php 

/**
 * Modify "Continue Shopping" button URL on the Flux Thank You page.
 *
 * @param string $shop_url Shop page url.
 *
 * @return string
 */
function flux_checkout_change_shop_page_url( $shop_url ) {
	if ( ! Iconic_Flux_Core::is_thankyou_page() ) {
		return $shop_url;
	}

	// TODO change this URL.
	return 'https://site.com/my-custom-shop-page';
}
add_filter( 'flux_checkout_shop_page_url', 'flux_checkout_change_shop_page_url' );