Is there an expiation for what I did by seeing a video of my sister's friend without a head covering?

If you looked at a girl, whom it is impermissible to look at, unintentionally, and then averted your gaze, there is no sin upon you. However, if you intentionally looked at her with lust, then you are sinful, and the expiation for this sin is by seeking forgiveness and repenting to Allah Almighty.