A change log? See the change log link included in the footer of each download to go directly to change logs.
www.joomlapolis.com/downloads
If you mean a log on your site as to what files, database, etc.. changed there isn't one.
As for when a new release is available your site should already be telling you this in your Joomla control panel (you should have a module named "COMMUNITY BUILDER UPDATES"), in CB > Plugin Management (version will be red if out of date), and in CBs Install from Web. You can also check our releases page below.
www.joomlapolis.com/news/releases