Important: Any files here are ALPHA test versions only, please BACKUP your current system before testing!
To backup your existing v2 Poweramp settings and database you will need use a file explorer app to make a copy of the contents of the root folder: /data/data/com.maxmpz.audioplayer/
Then to roll back, first uninstall the v3 test release, then reinstall the older v2 release (from the Play Store or from the apk available on this site) and then restore the contents of that backed-up folder.
This can also be done automatically via an app such as Titanium Backup, which can save the whole app+data in one go thus allowing you to reinstall the old version with a single tap.
For more discussion on Poweramp v3 test releases, see http://forum.powerampapp.com/index.php?/forum/18-poweramp-v3-testing/