In order to find the updating file, go to Start, type
update in the Search Programs And Files field. Once you find it, right-click its icon and select Open File Location. If you know the downloading date of the update, you can type
DateModified:mm/dd/yyyy. If you can't find the update, I recommend that you remove the antivirus from your computer using Avast Uninstall Utility and download and install its latest version from the official website.