When you swipe to your home screen, tap an hold on an open area. The edit home screen should pop up. swipe left and you should be able to choose between Google discover and samsung free. It can also be turned off if o you'd prefer neither.
There is no option for Bixby anymore. Unless you find a way to hack the one ui home you are stuck with either samsung free or Google discover, or samsung adds the option for Bixby again.