How can probiotics update gut flora?

Variable per person: Your question is excellent, but the response is individualized based on a patient's goals with probiotics. GI transit time, diet, use of probiotics, co-morbidities that may require use of antibiotics or other medications that affect gut flora, as well as the underlying pathology found in the gut may all alter benefit. Typically your "back to normal" in 3-4 days w/lots of probiotics to choose from.