CURRENT STUDENTS must meet the change of major criteria as outlined at http://www.changeofmajor.uci.edu/. (some double majors might require higher standards) For additional information, see here.
Once the criteria has been met, simply apply for a major change online through Student Access.

NEW FRESHMAN STUDENTS may not be able to change their major this summer after July 1st. For more information about Physical Sciences majors, and/or to request a change of major BEFORE July 1st, please contact us at pssazot@uci.edu. 

It is always possible to change after you've taken some classes at UCI. There are specific criteria governing this process which your Academic Counselor can explain to you when you attend summer orientation. Visit http://www.changeofmajor.uci.edu/ for more information.

NEW TRANSFER STUDENTS cannot usually change their major once they have been accepted. An Academic Counselor in the office you want to change to can evaluate your previous coursework to determine whether or not you qualify for the new major. If you do, then your major can be changed. Visit http://ps.uci.edu/~stuaff/ac.html for Counseling office info.