You declare your major at the end of first year:
“Engineering courses are required in the first year, giving students an opportunity to familiarize themselves with a discipline before choosing a major.”
https://engineering.cmu.edu/education/undergraduate-programs/curriculum/index.html
“Since students in CIT do not select a major until the end of the first year, all first year students share a common experience consisting of introductory courses in the engineering majors (one each semester), calculus, physics, other science courses which compliment specific introductory engineering courses, and courses in the College of Humanities and Social Sciences (General Education). This curriculum helps make an informed decision about a final major.”
"http://coursecatalog.web.cmu.edu/collegeofengineering/#firstyearforengineeringstudentstext