...you have found your present day mind transported back into your body on that day. Assuming that you would try to stop the attacks of the following day (a choice that is a whole other topic in and of itself), how would you do it without ending up under suspicion of being involved in the plot, yourself?
The event in question doesn't have to be 9/11 - you can substitute any other major catastrophe you wish and place yourself the day before. I suspect, though, that this would actually mostly get easier the further back in time one went.
You'd be messing up the timeline and that's bad! If you travel back to time, you should try your best not to change history, and instead just observe!
But seriously, I would try to buy a ticket for one of the planes, try to sneak a weapon, and then try to stop the hijackers. I would inform the other passengers what the hijackers are attempting to do. I would also probably bribe someone to do the same on the other planes. That is if I have money.
I'll consider leaving an anonymous signal through different means about what's going to happen, however I fear that this might alert the terrorists and only delay the event. I also doubt that I can cause anyone to act on it that way. I might call the passengers and alert them to act.
However, that's a bad idea, since if I manage to stop the attacks, no one will take what could have happened seriously, and there would be no special measures taken to stop similar attacks, so the attacks would just be postponed. Which is one of the reasons why I would try not to interfere with historical events.
And after watching "City on the Edge of Forever" I hope I never have to make such decisions in my life. I prefer to do what I find the best not knowing what will happen.
Stopping one of the planes that went into the towers, however, would be a good thing. I think I can manage to do it myself.