https://wccftech.com/downgrade-tvos-13-gm-to-tvos-12/

We’ll show you how you can downgrade tvOS 13 GM to tvOS 12 on the Apple TV.

We’re really, really excited about the tvOS 13 GM update. It’s fresh out of the oven, packs some wonderful features including multi-user support and whatnot. However, if you updated to tvOS 13 GM and think to yourself that you might have made a mistake, then you will be pleased to learn that you can undo it all as long as Apple is signing tvOS 12.