Reporting GPAs from the same university
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.