Is there such a thing as that?
Why do you want to replace the existing flora? It’s bad flora?
Complicated answer. Either because of radiation, poisoning, disease, or antibiotics, the flora might be damaged or even pretty much imbalanced. Or, it might be having a bad interaction with an individual’s immune system.
Traditionally, a way of treating it was with what is called a “fecal transplant”, aka a “chocolate shake”, quite literally, a small amount of a healthy person’s feces put in a chocolate shake for the person with a poor flora to drink.
But this is a primitive way of doing things. Human flora contains some 300-1000 different microorganisms, though only 30-40 take up almost all the available space. And the presence or absence of any of these might be responsible for serious medical problems.