I got accepted to CSUSM and I major in psychology BS but I was wondering if I can change my major to another field because I am not sure if I want to major psychology it.
A lot of students change majors during their first year of university. It is very common for students to take introductory courses in several different subjects their first year, as part of deciding what to major in.
Some majors are restricted at some schools. You would be best off to ask admissions at your university. However, it is not unusual to start off with one major but with an understanding that other majors are worth considering.