How to ask forgiveness from ALLAH because of telling a lie?

I have heard and now read a hadith:-

“Can a believer be a coward? He(Prophet Muhammad (PBUH)) said, “Yes, he can be coward. Then it was asked whether a believer could be a miser? He replied, “Yes, he can be a miser”, it was then asked whether a believer could be a liar? He replied, “No, a believer cannot be a liar.” (Imam Malik, Mishkaat)
How to ask for Allah's forgiveness for this sin?
One is too weak to tell back a person that he was lieing,if the lie does not hurt the person in any way,still is it necessary to tell him the truth to ask for Allah's forgiveness?

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