How to open website my company in internal network

first make sure your web-server has a Static IP

second you will have to do a port-forward to the web-server

third does your main office have a Static IP?

if yes, all the branches have to do is type the IP and they should be able to connect.

if not, create a Dynamic DNS (DDNS) for your main office and your branches should be able to connect using your DDNS name.