Why always so serious and literal?
The most recent theory is that we could view the past, but have no actual physical impact on it.
That said, if I could go back...
#1 Before the Big Bang to see what actually was there before and what/who lit the fuse.
#2 The very start of man so I could implant a genetic message into him so that religion could never ever ever exist lol
#3 Last Thursday morning and keep myself asleep a wee bit longer as I was having a Very nice dream