Give credit where it is due: AT&T is on a roll right now. The US carrier was the first to release the Android Oreo update for the Galaxy S8 and Galaxy Note 8 last month and now have become the first to release it for the Galaxy S8 Active. The update has been released but as is often the case, it will be slow rolling out to everyone so give it a few days.
The update is R16NW.G892AUCU2BRC5 for those keeping score at home and it is about 1.2GB in size. That means you will want to make sure you are connected to WiFi to download it from AT&T.