Genre: Low Fantasy or Alternative History?
Question: What genre for an MC who gets a chance to live his life over again at a very young age, and decides to use his knowledge of future events in order to change world history?
Generally, alternative history describes fantasy stories in which the history of the story world differs from that of the real world. This may because certain historic events happened differently, or because magic exists in the story world and that has affected how history unfolded.
For example, Philip K. Dick's book, The Man in the High Castle
is an alternative history in which Germany and and Japan won World War II. The Bartimaeus books are about a world where magic exists and magicians became the most powerful people in the British Empire, serving as politicians.
The difference between alternative history and low fantasy is that in low fantasy, the fantasy elements have not changed the course of history (at least not in a way that most people are aware of it) because the magic or the fantasy creatures remain hidden from almost everyone in the world.
In some alternative history stories, time travel is used to cause history to diverge from the way we know it. The emphasis is on examining how the world has changed as a result. It's all about the "what if?"
If the time travel doesn't change history in the end and the story is about an adventure that takes place in a different time period, then I would call it s just a time travel fantasy. Ditto if it is concerned only with changing the lives of only a small number of people, or just the main character (as in the Back to the Future
If your story is all about creating a different world, I would call it alternate history. If it's just about the main character changing his life, I would call it a time travel book.
But don't take that as definitive. There are better scholars than I cataloging all the variations of the time travel trope.