Since I was never traumatized by the subject, and since I mostly had a good time, I would be hard pressed to want to change much. It was what it was and I learned a lot. Met a lot of cute girls--which has always been fun.
Lot of attractive and intelligent people who went through the Scientology mill. Glad I met some of them.
However, to answer the question, I'd lean over my younger self's shoulder and whisper, "You know those feelings you keep having about all of this? Those feelings that something isn't quite right? Trust them."
Which I did anyways, so maybe my future self came back to my past self and actually did whisper those things. Time being an illusion and all.
Now if I could just get that future self to whisper the numbers of the next huge Powerball payout, I'd take you all out to dinner.