The latest Samsung smartphone to receive the coveted update to Android 7.0 Nougat is the Galaxy Note5. Specifically, AT&T's version of the last high-end phablet before the ill-fated Note7 came and went.
Some owners of Note5 units purchased from AT&T have already installed the new software release, which is rolling out over-the-air as you read Samsung Galaxy S8 Specs
this. When you do get the notification, keep in mind that you'll have around 1.44GB to download, so maybe do that over Wi-Fi.
Once you apply the update, your phone will run Samsung S8 Clone Android 7.0 and build number NRD90M.N920AUCU4EQC6.
This comes with the March security patches included - too bad you don't get the absolute latest security level though. As always, the update's rollout is likely to be staged, so it may be a few more days before your particular handset can get it.