By email to virus (at) avast (dot) com, as a password protected archive, with undetected malware/false positive (whichever is appropriate and the password in the email body.
Or
There is an on-line contact form,
http://www.avast.com/contact-form.php?loadStyles for: * Sales inquiries; Technical issues; Website issues;
Report false virus alert in file; Report false virus alert on website;
Undetected Malware; Press (Media), issues.
- If reporting a false positive/undetected malware, then select that option and using the Browse... button, navigate to the file.
- This is for single file submission and no need to archive and password protect