The injustice or harm one may do against another differs, it may be material or moral. If it is a material injustice, then one has to give them their right back or to ask for his forgiveness in any way as long as one can reach them.
If it is a moral harm, we should differentiate between the moral harm done in front of other people and the harm done between the perpetrator and the victim only. As regards the harm done in front of other people such as backbiting them or insulting them in front of people, the perpetrator should mention the same person with good in front of those people and to seek his pardon if possible. They should also make du`aa’ for them and to be courteous towards him.
If no one witnessed the harm done against the person except both of them, then the perpetrator has to seek his/her pardon. If they did not forgive, the harm doer should ask Allah for forgiveness and to make du`aa’ for them in the hope that they forgive on the Day of Judgement.
Also, in all cases especially if the harm doer cannot reach the person, they can make du`a’ for the person and give out charity on his/her behalf asking Allah to guide him/her to forgive the harm doer in this worldly life or in the Hereafter.