You don’t specify what country or region you’re in, but I’ll take the easiest approach to start with.
Assuming that you have an Internet connection at each location, get a couple of SonicWall security appliances (other brands work just fine also, I just prefer these), and set up a VPN tunnel between the locations.
In many cases, you can tie a new security appliance to create a VPN tunnel with existing equipment on your headquarters end.
The key requirement will generally be fixed IP addresses on both ends, but it can be done (depending on make and model of equipment) with only a fixed IP address on the headquarter’s end. But this latter choice would require the remote site to always initiate the tunnel, since its IP would not be constant.