What changed in the end? --Warning Spoilers
Before he goes back in time, he manages to save New Jersey, but doesn't stop the 2nd Missile, and then California goes to shit and Lois dies.
Then he travels back in time, we see all the damage repaired in reverse, and at some stage he stops.
But did he stop the 2nd Missile?
On one hand it looks like he did because Lois is alive and there's no damage,
but them you get Jimmy Olsen turning up thanking superman for leaving him stranded in the middle of no-where in an earth quake.
Back in time scene: https://www.youtube.com/watch?v=LoCaeI5RffI