Obviously, the present cannot actually change what has already happened. However, the present can change how we think about what happened in the past and what we write about the past in history books.
As our ideas in the present change, the way we talk about the past changes as well. A few decades ago, before we were as culturally sensitive as we are now, textbooks would portray Native Americans as dangerous savages who had to be defeated so the West could be "won." Nowadays, history books tend to portray Native Americans as people who lived in harmony with their environment and were exploited and abused by white Americans. The past did not change, but the stories we tell about it have changed to be more in line with our present-day attitudes. In that way, the present can affect the past, or at least the way we perceive the past.