A Guide on How to Fix Avast Virus Definitions Won't Update - MiniTool If there is no pending update, you need to repair your Avast installation files. Step 1: Open Control Panel in Windows (viewed by large icons) and click Programs and Features. Step 2: Click the app and choose Change. Step 3: In the new window, click Repair, and wait Avast makes necessary changes to the data files to repair the client software.