Quoc: Toward the end of the season, Underwood orchestrates Walker's downfall. He secretly leaks the details of the money laundering, for which Walker is blamed. While publicly supporting Walker, Underwood works behind the scenes to have him impeached, with Sharp's help. In the season finale, "Chapter 26", Walker resigns, and Underwood succeeds him as President of the United States.

Answer this question based on the passage: when does frank become president in house of cards?
HW Chung: OK, some thoughts:
To answer the above question, the relevant sentence is: In the season finale, "Chapter 26", Walker resigns, and Underwood succeeds him as President of the United States. The answer is Chapter 26.