You’ve invented a one-use-only time machine. You can go back in time to any one day for 24 hours an do what you will, then you have to return to the future.
You go back to November 22, 1963 and wait in the Texas Book Depository with a shotgun. When Lee Harvey Oswald comes in, you blow his stupid head off and then go back to the future.
Would you remember the timeline when Kennedy was killed, or would you remember the timeline you created?
When you return to the present, you’d remember killing Oswald and why you did it, but nobody else would care that you had. He simply would have been some guy who was killed on the day Kennedy was in Dallas.
I think you’d have to learn the revised history.
Or would you remember both in a sort of confused way? Could that be why I can’t remember stuff like whether I fed the cats today?
I think you would remember your timeline, the “original” one, but you would have to learn the “new” one.
You have created a parallel universe in which Kennedy was not killed at that time. The present you return to stays the same.
No, no, no, no!
You’re not paying attention Marty! If you go back, eh, forwards, into the future from a point in the past, eh, present, where the timeline has been altered, you go to THAT ALTERED timeline’s future. You would return to a timeline where JFK was impeached after being caught on camera with three Vietnamese prostitutes and where Lee Harvey was some random Schmuck killed in a book depository.
Neither. You’d be stuck in Dallas, 1963 since your time machine is one-use only.
Not knowing the true nature of time, none of us can really know what will happen. This is sort of a pet peeve of mine with sci-fi time traveling stories. It seems that none of them really bother to explain what, in their universe, time is. By my count, time could be of several different types and affect you in several different ways:
Parallel universe
In this version, you will have created a branching timeline from the moment you altered time. Lets say every decision anyone ever makes can be divided into 2. From the moment you’re born, then, you would have been creating alternate universes where in one, you chose A, and in the other, you chose B. If you killed Oswald, you wouldn’t change the original timeline. Kennedy would still be dead, but you would have created an alternative timeline in which he wasn’t. Also, you may return to your original timeline, but in the interest of consistency, you would probably get returned to the future, but in this new alternate timeline. This is sort of Star Trek’s canon nature of time. They have an entire mirror universe that’s similar in almost every way except the people there are mostly jerks
One timeline, within time
OTWT postulates just one timeline with none of the crazy parallel universe stuff. You change it, break it, alter it, and it will distort this version of time, and you along with it. Unless you stepped outside time in your time machine, then you are not immune from it. When people talk about the “Grandfather paradox(ie. you can’t go back and kill your own grandfather because then you wouldn’t be alive to kill him)” they are suggesting that there is only one timeline and you are a part of it
One timeline, outside time
Like the above, except your travels through time makes you step “outside” time, meaning changes in it will not affect you. You’ll still be the you that went back to kill Oswald, and you could go ahead and off your own grandfather too. I suppose this has less to do with time itself than with how you managed to slip through it.
Time ripple
A few TV shows I’ve seen has this as the true nature of time. In it, time is either one or many, it doesn’t matter. What seperates this from the above is that the time traveler is essentially sticking to one timeline and can see the effects of time as it slowly affects things throughout history. Time is slow 
Changes you make in a timeline isn’t immediately obvious. You could kill your own grandfather, but in time, you will disappear because you don’t exist. As the flashpoint where Oswald is killed happens, it ripples slowly throughout time. The world changes before your eyes. If you had killed your grandfather, you’d see yourself disappear from family pictures, your wife being married to someone else, and how the world would be like without you. This is kind of dumb since it assumes that time itself has a speed limit, which is extra suspension of disbelief on top of more suspension of disbelief.
Static time
Is time like a movie? Movies are basically still frames sequentially viewed to give the illusion of motion. I don’t know what time is, but I have a hard time picturing everything we see as infinitely small increments of stillness layered and given the illusion of motion through some spectacularly cosmic energizer. Stephen King did this in the Langoliers
Of course there are small variations of the above. It could be a parallel timeline you’re creating in which case you’d go back to either the timeline you’re from in which Oswald still shot Kennedy, or through some invisible hand of creative writing you’d end up in the timeline you created. Maybe there’s only one timeline and if somehow Kennedy’s death indirectly causes your non-birth, you wouldn’t be able to kill Oswald. Or a fated timeline in which Kennedy’s going to die no matter what, and you could kill Oswald but a tree falls on him, or he chokes on a pretzel and dies. It would be a different timeline, but not one that would be markedly different from the one you’re used to. Sorry about the long rant, but time travel bugs the hell out me when used in fiction.
Basically, since you’re inventing a fictional time machine, you also have to invent the fictional dynamics of time causality.
It’s like asking if alien invaders would prefer to use Celsius or Fahrenheit for temperatures. The sticking point is the alien invaders. Since you made them up, you say what they’d do.
Ok…I’ll bite.
Today you go back to 1963 and kill Oswald. The changes from that ripple forward in time at the rate of 1 second per second. However, when you go back to the present, time hasn’t ‘caught up yet’ so it will be the original history. If you stay in 1963, you will see the new history unfold because you are traveling along with it.
So, if you wait 2 years and travel back in time to 1977, the original history will still be in effect. Travel back to 1964 and you will be in the new history.
Ok…I just made that shit up…but thinking about it…it does allow time machines and not cause paradoxes. You could go back and shoot yourself but you will continue to exist because the effect of you not dieing will never catch up to you 
I personally like the parallel universes model…but I do think you would return to the altered timeline like Dr. Brown said 