Om Swami
I did something wrong by mistake. How do I atone for it?
Atoning for a genuine mistake
If you took an action thinking it was okay and later realized you should have done differently, that is a genuine mistake, and you must not hold yourself more guilty than you already have. If you can still meet the person you wronged, apologize. If not, do prayashchit (atonement).
If you can still meet the person you wronged, apologize. If you cannot, there are ways of prayashchit (atonement), which he has described in his book on the ancient science of mantras.