There's no reason to let Google know, Google just knows as it is. Their "web crawlers" or "spiders" crawl the "world wide web" constantly.
Whenever you add content, change content, or improve content on the web - Google figures it out, and I mean quickly.
Whenever I find something to improve on a hub that I might have published a year or more ago? Google sees it within 24 hours, as the next day there is nearly always if not always a little red arrow to let me know that traffic spiked either some or plenty on an older hub.