In my own thoughts on this topic some time ago, I was first thinking what the current me can teach the younger past-me.
Skipping several logical iterations, finally I had come to the scheme when the current-me contacts the past-me, and both personalities get merged.
(In my case it would be easy, as the current-me is mostly a better informed and experienced past-me with useful post-knowledge.
So, the young past-me would just feel that he has just loaded a HDD copy of intenet, if that me could know what's the "HDD" and "internet", as it's 70s-80s.)
So, then I just was following the "my totally current personality inside my that body learning at school and then in the university" scheme,
The post-knowledge (i.e. memories) stays with me (otherwise I would just be repeating same decisions and sometimes mistakes), so watching in 1985 the launch of Columbia, I'm aware of the 1986 Challenger, at it happens in its time.
But I can't just recall lottery tickets, footbal matches, or so, because they are blurry background events for me, implemented randomly.
Also I can't remember exact dates when I, say, caught a hit in face because was walking that exact route at that exact moment, not five minutes sooner or lateer. I just don't remember the date, so I can't choose another time or route at that exact date. (Though, I can unintentionally do this by random, so occasionally pass that street not simultaneously with that bad people, walking there at that exact time).
I just remember this. I'm aware that I'm surfing that reality not first time.
Go and tell about? No, thanks. Nobody would believe before, but everyone would accuse after. So, it would save nobody, but just increment the casulaties counter by one.
Actually, that's the strongest reason to avoid showing off with the post-knowledge in that second-hand reality.
Just technically I'm so far from buying google stock in the late USSR, that my ideas were limited with companies known to me, and how would I optimize my career and so.