there are 2 main theories on changing the past:

1) if u go to the past, and kill your dad, you will not cease to existence, however you have created a new dimension whereby on the new dimension, u didn't exist.

2) The doreamon theory, where if u go to the past, whatever u try to do, u actually didn't change anything, as you are part of the past itself.
For example, u went to the past to make sure hitler will not start the WW2, but eventually u realize, by going to the past and trying to change history, u are actually the person who made the history, it is because of u, the WW2 was triggered. ur not changing it, but u r part of the destiny. Time and space is just a straight line.

The doreamon theory seems to have more sense.
Have u guys watched back to the future? The director seems inconsistent on the theory that he implies.
There's a scene where MJ fox slowly dying because he went to the past and triggered something that made his young mom and dad wasn't together. And while in the midst of dying, he had to change the destiny to make sure his parent's are together again.
That was totally doesn't make sense. If he triggered the separation of his parents, and by doing that he will not exist and he will not have the chance to go to the past and trigger the mistake he did. If you get what i mean.

it was proven on an experiment that the faster the object, the slower the time inside the object.