Silver Demon King is a Manga/Manhwa/Manhua in english language, Action series, english chapters have been translated and you can read them here. You are reading chapters on www.mgeko.cc fastest updating comic site. The Summary is

Han Yu-an, a senior in high school, failed the SAT exams as he accidentally shifted down his answers on the answer sheet. Before him appears a handsome foreigner... no, a demon, who says he could turn back time for Han Yu-an. However, he would have to act as the stand-in for the Demon King... Can Yu-an safely return home after having this abrupt change of lifestyle? "Should I just live however I want? Because until the real Demon King returns, I'm the actual Demon King!"