You have to take it to the dealer to get this update. They do that so they can tell you that the update is not compatible with 2015 model year Tahoes. Even though if you have the 8 inch Mylink panel in your 2015 Tahoe, it has the exact same hardware that the 2016 model does.
But they wouldn't sell as many 2016s if they let 2015s upgrade. They'll come out in the Fall and announce they've magically come up with a work around to apply the update to 2015s once a few aftermarket techs successfully upgrade them. That'll also be when they are done selling 2016s...