How can money acquired through illicit means be returned to its unknown owners? Is it necessary to increase the amount when returning it? And what are the ways to repent from this sin?

You must offer sincere repentance for the sin, by regretting it and resolving not to return to it. If the sin relates to the right of another human being, then the right must be restored to them if possible, and there is no obligation to restore any excess. If they forgive you, you are absolved; otherwise, the right must be restored to them by any means. If you are unable to reach the rightful owner, then give the money in charity on their behalf for the benefit of Muslims or to the poor and needy.