How does one repent from denying a Sunnah of the Prophet, may Allah bless him and grant him peace?

If a person denies a Prophetic Sunnah out of ignorance, then he must affirm it once he learns of it. Indeed, some Companions retracted previous fatwas when the Sunnah became apparent to them, such as the retractions of Umar, Ali, and Ibn Umar in some jurisprudential matters. However, if he knows the hadith and denies it, or does not consider it a proof, then his matter is grave, and he must repent and believe that the words of the Prophet, peace and blessings be upon him, are binding proof.