Here is how you do it,
Connect a USB drive with a System containing Internet connections
Download the Update from the official site.
Check the version of Avast you are using beforehand and the following update you are about to download
Once you've downloaded the VPS update file or the vpsupd.exe, make sure to copy the same into the USB Drive
Connect the USB drive to your System and past it, in any suitable folder/ drive you deem fine
Tap on the .exe file to finish the installation of the software