If you're asking how it chooses what league you promote or relegate to, it's completely random. As to how you promote or relegate, if you're a league champion, you're guaranteed to promote, however, due to teams going bot, you can still promote if you have a good record and point differential even if you aren't the champion. For relegation, if you're the 8th seed in your conference, you automatically relegate, and if you're the 6th or 7th seed, you play the opposite ranked team from the other conference in a best of 3 relegation series (for example, if you were 6th in the Big 8, you would play the 7th team in the Great 8).