When you say "Since their computers can't be connected to the internet, my way of updating their avast is through manual update."
Presumably since they don't have internet connections, you are downloading the complete virus definitions file from
http://www.avast.com/download-update and trying to install it on their systems ?
If so you are most likely bumping into the avast self-defence module, hence the file system error error 5 Access is denied.
So to do that you would have to disable the avast self defence module and re-enable it once complete. I don't believe there is a way to do that using a batch file/script.
As the System Administrator, I don't know how large your network is, but I would have thought that the ADNM would prove a better option as that would manage the other systems and they can be set to get their updates from a mirror, a system that has internet access that you download the updates to.
Sorry I can't be more helpful on the ADNM process as I don't use this application, you could check it out here,
http://www.avast.com/en-gb/distributed-network-manager