Update Samsung S7 overseas

I hate to throw cold water on the plan, but it really wouldn't matter if you used a VPN that would make it appear your phone is in the United States. 

Updates for an AT&T branded Samsung Galaxy S7 are only available OTA via the AT&T network.  In order to connect to the AT&T network the phone must be equipped with an active AT&T SIM card, e.g., you must be a current AT&T customer.    

As @Lilybell2 wrote, not going to work.  In the USA you could have updates flashed at the Samsung desk at Best Buy.  The alternate is OTA by having ATT SIM card and service.  You got zip.