In Stony that can vary greatly. If you're switching between majors in the same college—two majors within the College of Arts and Sciences (Biology to Biochemistry) or within the College of Engineering and Applied Sciences (Computer Science to Electrical Engineering)— it can be quite simple. Typically for this, you have to fill out a form and email an advisor and this can be done with little headache. I know people who have changed their majors 2+ times since beginning (though I wouldn't really recommend that).

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Now if your switch is interdisciplinary, this is more complicated. In your case, going from Biology to Computer Science would require you to go from the CAS to CEAS, and to do that here you'll have to take the required introductory courses then apply to be accepted into the major. The required intro classes are CSE 114 and CSE 215; CSE 214 is taken by many students alongside CSE 215, but if you go this route then prioritize taking the classes in a manner which would allow you to perform best.