Once you share your URL to your wall, Facebook caches your graph data – title, description, image, etc. – and makes it very difficult to find how to clear or update that data. The trick to doing so seems to be the Facebook Developers' Debugger tool. You can use this tool to preview the data Facebook receives, as well as to reset the data in Facebook's cache.
Go there, then enter the URL to your page or site. The debugger will show you the associated data, and the next time the link is shared on Facebook, it should crawl your page anew to update the data in cache.