Gao Zhan is Laojiu. How did he become emperor?

Gao Zhan was handsome when he was a child and was deeply loved by his father Gao Huan. During the Yuan You period of the Eastern Wei Dynasty, he was named Duke of Guang Jun; In the early years of Tianbao in the Northern Qi Dynasty, he was promoted to Wang Guangwang. Together with his brother Gao Yan, the king of Changshan, he planned to kill Yang Wei and others and abolish the emperor Yin Gao. After Gao Yan succeeded to the throne, Gao Zhan became the right prime minister, in charge.

Soon, Gao Yan was seriously ill, and Gao Zhan secretly met with his nephews Gao Yuanhai and Gao Guiyan, preparing to send troops to usurp the throne. The wizard divined: "unfavorable is quiet, favorable is auspicious." Gao Zhan didn't usurp the inverse power. Gao Yan died in order not to let his son fall into the fate of Yin Gao for a hundred years (Yin Gao is the son of Gao Yang, who succeeded to the throne because of his father's death, and Gao Yang ordered Gao Yan to assist him before his death. Soon, Gao Yan decided to pass it on to his younger brother Gao Zhan by launching a coup and killing his nephew Yin Gao.