I assume these branches are geographically separated (in different buildings/towns/states/whatever). In such a case, you will need what is called a "Virtual Private Network", or VPN for short. There are a lot of devices and software that will let you set up such a network, allowing your offices to connect as though they were all local. There is open source software such as OpenVPN that is widely used, as well as many "appliance" systems that provide such capabilities. Which sort of VPN you use depends a lot on how sophisticated your IT people (assuming you have any) are. If you are still clueless, check with your internet service provider(s). They may have VPN services you can utilize, for a price.