Yes, you can change your branch from Mechanical Engineering to Computer Science (Cyber Security and Forensics) after your diploma.

Many universities offer a Bachelor of Science (B.Sc.) degree in Computer Science with a lateral entry option for diploma holders. This allows you to join the second year of the program, potentially saving you time and money. However, the specific eligibility criteria and entrance exams can vary depending on the university.
Some universities offer Bachelors of Technology (B.Tech.) degrees in Information Technology (IT) with a specialization in Cyber Security or Digital Forensics. While not strictly computer science, these programs provide a strong foundation in IT and relevant security and forensics skills.

https://www.careers360.com/colleges/peoples-college-of-forensic-science-and-cyber-security-aurangabad


I hope it helps!