If someone might have said lashon hara by mistake unintentionally and quickly said something good, and the subject passed away. How can one do teshuvah?
Chofetz Chaim (laws of L.H. 4:12) makes an important distinction between L.H. that actually eventually causes damage to the person one spoke about, and L.H. that never leads to damage. In the latter case, one needn't ask forgiveness from the person he spoke about; it is only bein adam lmakom, and the teshuvah process is the same as for any other bein adam lmakom.
In the former case it is indeed also bein adam lchaveiro, so the teshuva process would be the same as for other aveiros of this type when the victim has died: one must visit the victim's grave etc. (for details, see Shulchan Aruch O.C. 606:2 with M.B.)