Your microbiome changes daily with the foods you eat, so eating more fermented foods like kimchi, sauerkraut, kefir, yogurt, kombucha all help add good bacteria to your gut.
Oftentimes, a probiotic supplement is also a good option. As an Integrative & Functional medicine physician, I often recommend patients take a probiotic supplement daily to help add good bacteria into the gut, and also aid with bloating, gas, and constipation.* Fermented foods are great, but your body often needs more diversity than what you can get from those food alone.
Within 24 to 72 hours of taking an oral probiotic, your gut starts adapting to the probiotic you are taking. For some people, results are instant; for others, depending on their existing microbiome, they may notice difference later. However, within two weeks, most people notice some improvement.