It’s very easy to switch majors, and it can all be done online. The only problem is that if you’re an incoming freshmen I don’t know when you’ll have access to that tool (you can use the Major/Minor tool https://students.ucsd.edu/academics/advising/majors-minors/declare-or-change-a-major.html). You can try now, but if you can’t access it, don’t fret. There’s nothing wrong with waiting until classes start to switch majors.