Firstly, there is a kind of time machine even now, this is a banal movement of everything in the universe, which is perceived as time, and a person, theoretically, can accelerate this movement personally for himself, and nothing will happen. It is already difficult to slow down the movement, there are no real ways. Reversing the movement is simply not realistic. Those. to do so that the time would go back 1 minute, this means that it is necessary to restore the state of the entire universe to minus one minute ago, but this is not realistic, since there is simply no such amount of energy, and if there was, then nothing would have happened , a new, different reality would just appear, with a different, slightly different, state of elements in the universe at every minute of time, that's all, and the old state would not even have a description, it would simply be absent in the new reality.
Nothing will happen in our branch of reality, users of a time machine will only create an infinite set of branches of alternative realities and will visit there to live, or will create tourism for moneybags in "parallel worlds"