[xwiki-users] XWiki upgrade

Hi, I am a user of myxwiki and I receive notifications on pages such as "June 6: Upgraded xwiki to version 3.1 SNAPSHOT from 2011-06-06. Don't forget to upgrade your wiki with the latest XAR (Upgrade instructions)." Does that mean the XWiki Core has been upgraded and I just need to upgrade wiki documents? If so, having downloaded the latest XAR (xwiki-enterprise-wiki-3.1-20110824.095236-263.xar in this instance) to my PC, can I expand it within myxwiki or unzip it on my machine? Do I export my xwiki, merge my files into the new build and then import back? Since with myxwiki the wiki is already available I don't have the experience of building one from scratch and the confidence to just make changes knowing they are on a local machine. Please can you provide some clearer guidelines for users of myxwiki. Thanks, Jonathan