President Kennedy was killed when he was 46 years old. He had been president for 33 month. History would be different if he had lived. He probably would have been reelected and served as president until 1969.Lyndon Johnson would likely have never been president.
There were a few American advisors in Vietnam before Kennedy was elected but the were greatly increased under Kennedy. The official start of that war is 1961. Kennedy was determined not to let South Vietnam fall to the communists. He would have probably increased the American troops there. Would he have done enough to win or did much the same as Johnson did? No one knows.
Civil rights bills had been sent to congress by Kennedy and were basically going nowhere because of obstruction by southern democrats. They probably would have been passed but not until after the 1964 elections,maybe even a year or two later.
How these two issues developed would have determined how politics and social issues developed in the late 60's and seventies. No one can say for sure but very likely things would have been different, maybe better, maybe worse.
Rest in peace president Kennedy. You will be remembered.