How do I get a software update on a refurbished phone still connected to its old carrier?

[Android Central Question]

I bought a refurbished Samsung Galaxy S6 on Amazon and realise that it was first used on an AT&T service. Though I know for a fact that there have been several software updates after the one I'm currently using, my phone does not offer any software updates in Settings > System > About device > software update

How can I force my phone to get the latest update?

I have never used AT&T and never planned to (my current carrier is French), so getting the software update by disconnecting from AT&T would be ideal, if possible.

Thanks!

 

[Rukbat]

You can't "disconnect from AT&T", you're not connected to them, the phone was just made for them and runs their firmware - and there's nothing you can do about that. If you look around https://forum.xda-developers.com/att-galaxy-s6 you may find that someone has captured an update package (they normally disappear from the phone as soon as the phone gets updated) and you can install that, or you can install the entire updated firmware - but that wipes the phone so you'll have to back everything up first (see Backing up an Android Device), then restore it after flashing the ROM (see [Samsung] How to flash Stock ROM via ODIN).