Occasionally, we'll roll out an update to a theme to fix bugs or add new features. When this happens, you'll be able to download an update either from our store (if you purchased directly from us) or from the Shopify Theme Store.
Unfortunately, Shopify does not provide a way to automatically update your theme or any kind of upgrade feature. The only way to update a theme is to either install a new version or follow a series of advanced steps to update the templates yourself. I realize that this is frustrating to a lot of people. Believe me, if there was a way to build in an auto-update script or something, we certainly would.
Here is an article on Shopify's support site that describes the process of updating your theme. Please read this article carefully before performing a theme update. If you have questions, please don't hesitate to submit a support request.