Only if time travelers from the future go back in time. For now though we think time travel is only possible in ... ONE DIRECTION ... (that'll get the little girls' attention haha) and that's forward. Astronauts on the Space Station ate actually traveling through time right now, but by only a fraction of a second. The faster you travel and the further you get from earth, the more you will time travel into the future.
Well, technically speaking yes. Your present can change your past by having a whole new life! You can forget about your past ex. old friends. and go on with the present ex. new friends. The effect on it all would be change. If you did go into the future (impossible) and came back to the present. Whatever you saw or did would effect the present, because you would tell others what happened, and if you tell the right people. Then, the right things will happen. Like for example, if you went into the future (might I add crazy/impossible) and they had an iPhone4S9. If you cam back to the present and told people; if you tell the wrong person then they might create it. Then you go to the future and it would have changed because someone in the present already made it! (Sorry for all of the blab, I have a lot to say!) (P.S. I hopeI helped!)
Neighborlady is right, it can only change the way you think about the past present or future. For example: your present more informed thoughts may currently affect how you now think of the past which may influence what choice you make or view the future.