You can't change the events that happened in the past, but I guess you can say you can change what's happened, by taking an action to change it... For example, me buying something yesterday will always have happened, and I'd have that item in the future. However, I can return that item, so that it's as if it didn't happen.. If that makes sense at all... That is assuming there would be negligible cost to taking any specific action though, because as soon as there's a cost, then obviously, by making that action to change what happened, gives another effect which might not necessarily be irrelevant. Not sure if that makes any sense.