How do I tell my Apple TV 4K to retry for using 4K as a default? When I set up the Apple TV 4K, it was possible that the Internet speed was slower than usual, and Apple TV 4K said it is not possible to play at 4K for the iTunes movies that I purchased.  

But what if I stop all other Internet connections, and want Apple TV 4K to re-try to see whether it can use 4K as a default (or that I don't mind the movie being choppy once in a while), how can that be done?
The Apple TV estimates bandwidth available every time you play a movie - so it is done automatically. There's no need to rerun the setup process whenever the load on your internet connection changes.