Teshuva for Kabbalas Loshon Hora
If one transgressed kabbalas loshon hora by believing negative information, and he wishes to do teshuva, he must cease to believe that the information is true, in addition to vidui (confession) and accepting upon himself for the future not to believe lashon hora again. By doing this, he will rectify the negative and positive commandments that he transgressed with his acceptance of the loshon hora.
Transmission of Loshon Hora
If he already relayed this information to others, he must either convince the listeners to disbelieve the report or beg the subject for forgiveness, as with any other case of speaking loshon hora. (Hilchos Loshon Hora VI, 12, 34).
No Transmission of Loshon Hora
In the case of Kabbalah alone, though, one should not ask for mechilah (forgiveness). One who only listens to and accepts loshon hora, but does not pass it on, does not necessarily cause pain and suffering to the subject. Moreover, by asking mechilah he may cause the subject to feel hurt or anxious if he was unaware that he was being discussed. This may also lead to rechilus if the subject tries to find out who defamed him. (Nesivos Chaim, Shu"t L'Chafetz B'Chaim, Shiurei Iyun-Dirshu pgs 397, 617)
Source: The Laws of Loshon Hora and Rechilus, bit.ly/loshonhorabook