Do I need to go to the Public Security Bureau to write a letter of understanding?

No need. There is no legal requirement to write a letter of understanding in the Public Security Bureau. The letter of understanding can be written anywhere, but it is best to sign it under the witness of the public security organ, which can enhance the effectiveness of the letter of understanding. A criminal letter of understanding is a legal document reached between the victim and the suspect or his family in a criminal case, which is used to show the victim's understanding of the suspect, thus playing a role in mitigating punishment when making a judgment. As long as the contents of the letter of understanding comply with the law, the criminal letter of understanding has legal effect even without the consent of the public security organ, as long as the parties voluntarily and legally sign it. To sum up, you don't need to go to the public security bureau to write a letter of understanding.