offline update

Is there any way to do an offline update of avast mac security?

Hello,
no, you need internet access. In current threat environment, where new variants of threats appear each minute, scanning engine requires constant stream of updates, so it does not even make sense to try to accomplish something like this.
Kind regards,
Ondrej Kolacek

Install Avast when you have internet access (Such as public hotspot) and it will stay running. The next time you have internet access it will download the new updates automatically.

Since most malware is distributed through the internet, you risk is significantly less when offline