Talk to your college adviser to learn about the rules regarding changing courses. You only have a certain amount of time to drop a class after the semester starts. This time period is stipulated by your university.
Choose a new major or course of study, or classes that are available for you to take that interest you. Typically, you don't have to declare a major until your junior year of college. Keep in mind that if the semester has already started, the list of available classes may be very short.
Sign up for the classes and follow the appropriate university protocol for dropping existing classes. All universities will vary on their course changing protocol. Ensure that you consult your college adviser to perform the task correctly.