This may be a problem of the servers, announcing the existence of a new version with mixed numbers. But if you always used a stable version of avast, it shouldn’t happen at all anyway. If you ever installed a beta version, then it might.
Either avoid updating yet (since it is still in beta), or uninstall avast, clean the installation with http://www.avast.com/uninstall-utility under Windows Safe Mode and install the latest stable version of avast.
Since the new stable version is only a couple of days away, I would suggest waiting, and then clean up your current version before installing the new stable one when it is released. (Note: save a copy of your avast license file before uninstalling.)