Have you tried this:
Manual update of VPS - avast! Virus Definitions Update - Manual Download VPS Update, ensure you download the correct version avast! VPS update for version 2014 (virus definitions, vpsupd.exe, approx 39.1 MB). - Save the file to your hard disk, locate where you saved it and double click it and that should carry out the virus definitions update.

If this is the one you have tried, what errors are you getting ?