If we commit some wrong unintentionally then how do we do penance for it?

Sincere repenting in one’s own heart, by itself is true penance for sins and wrong doings committed knowingly or unknowingly.  Ritual itself is not important in penance.  Heartfelt repentance purifies the soul.

Singing in hymns in praise of God eliminates sins committed unknowingly.

Nām sankīrtanam yasyā, sarvapāpapranāśanam – Shrimad Bhagwat, 10^th canto, Chapter 13

Ajaamil performed penance in this manner and rid himself of all his sins.