[Angelino97]

One theory I read is that if you go back in time and change the past, you're really creating another universe in the multiverse.

So if you save Lincoln, then Lincoln has still been shot in your universe, but you've created a new universe in which Lincoln has not been shot.

Something to do with string theory, I think.

[mdmathis6]

One such writer called it “creating a stem” where the previous time line will continue but a new timeline where the alteration occurred continues from that point. Thus you can travel back to a future that still has you in it but the other time line has another you.