My low-information read of the current state of knowledge is: we've established at very high confidence that the gut microbiome really is very important.
And that's it; we don't know anything more detailed than that.
There's also a probiotic called visbiome (formerly* known as vsl #3, which is the name the research literature uses) that has shown a lot of effectiveness for IBS and similar issues. I don't feel like digging up the studies for that unfortunately but there's quite a bit of evidence.
* vsl #3 is still marketed but is supposedly a different manufacturing process now. thus visbiome is now the "real" one
But intuitively eating unprocessed foods that are not made with artificial or chemical anything (including artificial sweeteners) is probably a good start.
Fasting is probably another good way to reset your gut.
I don't think "resetting your gut" is a good way to think about it. Taking oral antibiotics will "reset your gut", in the sense of killing most of the existing biome and leaving space for new flora to grow into, but that can lead to problems as the biome that develops from a clean field isn't the same as the standard biome. To the best of my knowledge, you normally inherit your gut flora from your mother during birth and nursing, and then your gut hopefully stays stable over time. "Resetting" is more likely to give you a worse biome than a better one.
Here's a random study about vaginal microbiota:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4178329/
There's also a probiotic called visbiome (formerly* known as vsl #3, which is the name the research literature uses) that has shown a lot of effectiveness for IBS and similar issues. I don't feel like digging up the studies for that unfortunately but there's quite a bit of evidence.
* vsl #3 is still marketed but is supposedly a different manufacturing process now. thus visbiome is now the "real" one