I have graduated with a BS in Computer science degree with 3.6 GPA from university of X. Then after graduation, I continued taking classes in different program (Biology) but didn't do well and my GPA went down to cGPA of 3.4. So, the gpa on last page of my transcript (cgpa) is 3.4, but also on the transcript it mentions that I have earned a BS degree in computer science with 3.6 gpa. My question is that if I want to apply to a graduate school in computer science program which GPA I should be reporting? Would it be 3.4 or 3.6?
Ideally you should report them both. Online forms might not let you do that, however. If you can only report one, I'd suggest that you list your CS GPA but put a note somewhere detailing the other. You want to give the most complete information so that no one can suggest you have been dishonest.
You will probably have to explain, somewhere, why you abandoned biology for now and simply saying it was because you didn't do so well would, IMO, be a mistake.
However, your university might, independently, give your GPA, in which case you should know how it will be reported by them. If they combine them, then you probably need to do so also, but explain in your application materials the history of it.