Add the sample code below to your child theme's functions.php file or in a site specific plugin.
<?php
//* Do NOT include the opening php tag
//* Replace free event no billing messaging with something else
function ee_replace_free_event_no_billing_messaging($content)
{
$content = str_replace('This is a free event, so no billing will occur.', 'This is a free event. Please click the Finalize button below to finish your registration.',$content);
return $content;
}
add_filter('the_content','ee_replace_free_event_no_billing_messaging', 200);
References: