Well, you are obviously referring to the standalone aswClnr.exe.
It is a simple removal tool - there is no automatic updating implemented (and it never will be, I'd say). So, to update it, you simply have to download a new version.
In any case, this tool should be used only when you already got infected - and it shouldn't happen very often if you're using avast!
So, there's no real need to keep this tool updated all the time (from your point of view; when you really need it, you can always download it).