When you re-enroll at the end of each term, the University Enrollment Guide appears. Your current degree is automatically highlighted. To switch majors, simply pick a different one from the list. If you want to change schools, click the back button to return to the Select a University screen.
https://i.imgur.com/NRexfhM.jpg
If your sim is already in the middle of a term, you'll have to use the phone/computer to withdraw or drop out (or just wait until the term ends.) Withdrawing has a 2 day waiting period before you can re-enroll, but you keep your credits (although you will lose some when switching majors.) Dropping out has a 5 day waiting period and erases any credits your sims have earned so far.