11/22/63 by Stephen King

"On November 22, 1963, three shots rang out in Dallas, President Kennedy died, and the world changed. What if you could change it back?"

Pared down synopsis: A man travels back in time in an effort to change prior events, including the assassination of president John F. Kennedy.

Description (from Amazon.com and rather spoilery): "Jake Epping is a thirty-five-year-old high school English teacher in Lisbon Falls, Maine, who makes extra money teaching adults in the GED...