The Piwik Changelog page lists all recent Piwik versions and the list of changes.
The best way to keep track of all new versions is to subscribe to the Changelog Rss Feed. You may also register to our Changelog newsletter here to receive an email whenever there is a new Piwik release.
To stay tuned about the latest Piwik developments, follow the Piwik blog (you can subscribe to our rss feed). If you are interested in technical matters, you may follow our public issue tracker to check our progress, and register with the piwik-hackers mailing list (more info in the developer section). You may also register to our newsletter to receive an update from us a few times a year.
When a new version is released, your Piwik will display a message inviting you to update to the latest version.