How to change major?

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.

Similar question, how does one change degree and/or college? Am trying the college thing for first time here with Lilith Pleasant, and no way am I doing villany. Have changed her to a normal sort so want to take her through a normal program. How do I do this?

Thank you! Near as I can tell the sim hasn't started the actual process of classes or other work yet. Clicking the mailbox has only the never been there yet options. So use the phone?

Yes. You can use the phone, computer, or mailbox. Don't forget to apply for scholarships if your sim needs it.