Ciao Fello,
non puoi richiedere nessun aggiornamento, in pratica funziona come quando Google crawla il tuo sito, dipende dalla frequenza con la quale il GoogleBot visita il tuo sito o la tua pagina.
Ti incollo un poco di Q/A da Google:
Q: How often are previews updated?
A: Previews are generally updated regularly or re-generated on the  fly when requested by users. In some cases, it’s possible that preview  images are not updated as frequently as we normally crawl and index the  page. It is not possible to manually request an update of a preview  image.
L'unica cosa che si puo' fare, **VOLONTARIAMENTE **e' bloccare la preview:
Q: How can I block previews from being shown?
A: You can block previews using the "nosnippet" robots meta tag or x-robots-tag HTTP header.  Keep in mind that blocking previews also blocks normal snippets. There  is currently no way to block preview images while allowing normal  snippets.
Qui il link completo con anche altre domande:
http : // sites.google. com/site/webmasterhelpforum/en/faq-instant-previews