"What's done is done and can not be changed." - This is only half true. While what's done is done, it is only our memory of events that become our past, and how we choose to interpret them drastically determines our now. And while we can not change the actual events, or their effects on others, we CAN change the meaning of those events in our lives. Allow me to offer this example:
A relationship comes to a very unhappy ending. We feel hurt, abandoned, betrayed....etc. All of our negative emotions, anger, and fears are rolled into 'bad memories' of that person, where even a mere mention of places, phrases, and songs become a painful reminder of this disturbing time from our past. Painful scenes replay of all the plans that will never happen, those dreams of a shared future, collectively crucified with a single word: Goodbye.
This emotional state can persist for months, years, or even lifetimes.
But for our example, let's assume we meet someone new. And they are even more wonderful and we're even more in love ... etc. Something magic happens in our mind that sounds a little something like this:
"If it were not for that past relationship ending as it did, I would have never been brought to where I am now ... and looking back, I wouldn't have wanted it any other way."
And suddenly those same memories, those same songs, all of those same past events ... no longer hurt. The events themselves have not changed, yet, the meaning we give to them has, and now, everything feels 'better'. (Ever been there?)
I'd like to point out that it is not our new relationships that change our past events, though we tend to use people in this manner to achieve that purpose, but the moment we change the meaning of those same events we immediately experience a different now.
And this is how we change our past.