As @darling1002 said, it's being released now! Carriers release updates in waves, so if you don't see it available today, take heart - you should get it soon!
This particular update is 1.4GB, so be sure you have plenty of space available (I recommend at least 3 GB), you have a strong Wi-Fi connection, and your phone is well charged.
How about the S6 Active for AT&T? I went to AT&T Tuesday and they told me they have no news and that I should look on Samsung's website for info. I can't wait to get the Android Marshmellow update but I have been waiting since April when I first got my phone.
AT&T isn't announcing whether they will or when to expect an update. For a communications company they could communicate better with their own best customers. Meanwhile, Samsung must decouple updates from carriers or risk losing to the big Apple machine.