Why did lawyer Chen change the weapon to save the suspect?

To protect his innocence. In the movie Save the Suspect, lawyer Chen was accused of killing a woman as a suspect, but lawyer Chen insisted that he was innocent. In order to prove his innocence, lawyer Chen began to investigate the truth of the case. During the investigation, Mr. Chen found some clues. By changing the murder weapon, Mr. Chen hopes to divert the attention of the police and let them expand the scope of the murderer's search to other suspects, so as to gain more time and opportunities for themselves to find the real murderer.