That's a difficult question because it is not good to fix things so big like the second world war. without that war, technologies wouldn't be that advanced like now.
So I don't think I will change anything if I could.
Yes, it is very bad to change big historical events, especially fixed points, but if i could fix something, i would make 911 (The crashing of the planes into the twin towers) not happen because I know a couple people who where killed because of it. RIP