T-Mobile's Samsung Galaxy S8 and Galaxy S8+ receive security updates
When you receive the notification telling you that your handset has received the update, do not continue with the installation process until you connect your device to a Wi-Fi network. You also need to make sure that the batteries on the phone are charged to 100%.
Since the update is being sent out OTA, it should arrive on your T-Mobile branded Galaxy S8 or Galaxy S8+ very soon. If you're the impatient kind, you can manually download it by going to Settings > Software update > Download updates manually.
Don't get terribly excited. This is not the update to Android 8.0 that you've been waiting for, although we are getting closer and closer to the eventual release of Android Oreo for the 2017 Samsung flagship models.
source: T-Mobile (1), (2)