Sadly, if we could change it it wouldn't be part of the future and therefore we wouldn't see it. The future isn't set in stone, there are many possibilities. The things we see with precognition are things that are so likely that they can't be changed. I don't like it at all but that's the way it works.
However, I have had situations where I had a bad feeling about something and I warned someone I knew about it and they changed their plans and they themselves avoided the bad thing that happened. It still happened, but the person I warned wasn't there for it. I guess it's better than nothing right?
You can't change fate, but not everything is certain. In fact very few things are actually predetermined, that is why no matter how good you are at divination, sometimes the results will be confusing. That being said, if you see something horrible and are able to warn people you can feel free to try. I haven't had much luck maybe your experiences will be different.