Should I tell the person I harmed that I regret it, or what should I do?

If the utterance occurred by mistake, unintentionally, it is pardoned, based on the Prophet Muhammad's (peace be upon him) saying: "Indeed, Allah has pardoned for my nation their mistakes, their forgetfulness, and what they are coerced into."

However, if the utterance was intended without intending the meaning, it is a sin that requires repentance. If the slander is accusing a chaste person of fornication, it is one of the major sins, and its repentance includes the slanderer retracting their accusation and informing those who heard the slander that they were lying.

As for slander that is not related to fornication (such as backbiting and calumny), these are sins that pertain to the rights of human beings. Repentance from them requires ceasing the act, feeling remorse, and resolving not to repeat it, in addition to seeking absolution from the wronged party. If seeking absolution is possible without harm, then it is obligatory. Otherwise, the slanderer must seek forgiveness, perform many expiatory good deeds, speak well of the one they slandered, and supplicate for them and seek forgiveness for them in their absence.