Please note that this is a BETA release of Paint.NET. If you are at all worried about crashes and potential data loss, then please do not install it. Also, it will expire in 12 weeks; an update will be available before then.
This build is not available via the built-in updater (mostly because it just takes a lot of time to get that thing hooked up). Edit: I've made this available via the built-in updater. You can get it by going to Settings -> Updates, making sure that "Also check for pre-release (beta) versions of paint.net" is enabled, and then clicking on the Check Now button