This question is so tricky because we don't know how going back in time works. While many scientists have theorized about this idea, we still have no idea if going back in time and doing so little as stepping on a butterfly on accident could change the fate of the human race. But let's say time didn't work like that, and you could change major events without having too much of an effect on the future. If time was more like that, I would probably try to go back to as many places as possible, and warn people of scams. Most people would probably respond with wanting to stop a catastrophic event like 9/11, but these catastrophic events and what we do after them is what defines us as humans. If it weren't for these events, we would not have set up half the saftey measures we have today. Scams are relatively small compared to the grand scheme of things, so this shouldn't have too much of an effect on human history. It would save people money, but everyone would forget about it eventually and it wouldn't change too much.