let me ask you this:

I think I have a fair idea of how to update the data on the main
office PC once I actually get the data

My problem lies in

1 - How to actually create a connection, like a dial-up, or to
detect the presence of a connection

2 - Physically transmit the file(s) to the main office Pc, and...

3 - How to know, on the main office Pc that a file has been received.
Or, beyond that, how to know that the remote PC is in the process
of transmitting, and then doing something to receive.

Any ideas?