If one person went back in time to start over differently, would it change the whole world, or just a large amount of people?

it would really depend what that person does. like something small such as wearing a different shirt or waking up early can cause small ripples to something major like preventing 9/11

regardless of what you do, it will change your individual life most def .

Nothing for the rest of us would change, according to the infinite multiverse theory you would just create another diversion branch universe that would probably later re merge into the principle space time continuum.

I doubt that time travel would be of much use to alter history.