What is the method of repenting from criticizing a channel that later turns out to be reciting the Holy Quran, and is merely uttering the Shahadatayn sufficient to expiate for that?

Insulting the Holy Quran is a grave sin. If it was intentional, then the person who did it is an apostate, based on the Almighty's saying: "And if you ask them, they will surely say, 'We were only conversing and playing.' Say, 'Is it Allah and His verses and His Messenger that you were mocking?'" In this case, one must pronounce the two testimonies of faith and perform sincere repentance. However, if the intention was not to insult the Quran, then we hope that Allah will forgive, for the Prophet, peace and blessings be upon him, said: "Indeed, actions are by intentions, and indeed every person will have what he intended."