To all the people in here saying “I could warn them and help them prepare to take on the conquistadores” while the sentiment is good, the conquistadores weren’t the ones that wiped them out, it was smallpox and other diseases. So unless you can synthesize a vaccine, or just expose them to it then so they can recover population in a couple centuries, they’re doomed no matter what.

A heads up is not gonna save them from their real killers, smallpox, unless you brought it with you and give them a couple centuries to recover population and gain immunity.