How would you prevent 9/11 from happening?

HELL NO, and one of the teens in this movie suggested killing HITLER hell no to that too.

In most time travel stories the travelers are observers who are forbidden from changing time. Of course these are overly simplistic generic stories that do not account for the "butterfly effect" where minute changes become huge over time. In these stories usually not killing anyone or affecting major historical events is all that is needed to avoid changing time.

The fabric of time is arguably the most delicate thing in the entire universe. All events in the world are based on exactly timed coincidences which was expressed in this movie by the airplane crash caused because when they were siphoning power from the neighborhood to operate their time machine (I guess that car battery was not enough?) all the lights went out and a teenaged athlete, the best one on the basketball team was stumbling in the dark and had an accident that broke his leg when in the original reality there was no such accident. The game was canceled as a result since I guess a suitable replacement player could not be found in time. The original reality had his father watching him play and I am sure very proud of him since his team won but because the game was canceled anyway and his father was an airline pilot he did not request the day off like he did in the other timeline and the plane crashed killing several dozen people who did not die in the other reality due to a different pilot and different circumstances.

The fabric of time really is that delicate. You never know how tiny changes could have major effects so you should not screw with the fabric of time, not even if it seems altruistic and heroic and very moral like killing Hitler, Preventing the rampages of serial killers. No one or even group of human lives is worth the risk of messing up the space time fabric of the universe, you never know what harm it could cause in the long run.

In fact altering any historical event that predates your conception (like for me killing Hitler since I was born in 1972) would almost certainly erase you from existence. Changing history especially in such a drastic way would alter the lives of so many people that the probability of your 2 parents meeting and copulating like they had in the other reality is almost zero.

Hours before the planes take off, blow up the entrances to the wtc. Hope I have the attention of the authorities. Then call into the fbi informing them of the names of every hijacker, the weapons they used, and flight numbers. If given more time, personally start killing every hijacker.

As tempting as it is, I'd be inclined not to do it, all because of the ripple effect afterwards. 'Fixing' 9-11 may mean catastrophe happens somewhere else.

As far as killing Hitler goes, I'd be hesitant to fix anything before my birth, because the ripple could knock me out of existence.

At least take a trip to the small airport in Maine and do something to delay or stop the hijackers from flying to Boston in the first place.

Drive a cart-loader into one of the planes as it sits on the Tarmac...everything gets shut down and the FAA investigates. If you are on the tarmac of an airport without the proper badge, folk will be wanting to ask you a lot of questions and listening very carefully to your answers.

Would you be able to take anything with you say like news footage or newspapers with images of the attack?

Prevent Bush from being "elected".
Recount in Florida.

I would stop the NSA's 'Black Dawn' operation, thus thwarting 9/11.