Talk with your academic adviser about switching your major. He may be able to help you determine which courses you have already taken that will apply toward your new major.
Declare your major before completing 45 credit hours by going to the Advising Center and completing a declaration of major form. If you have not already declared your major, even if you are completing work towards one, you do not have to change it, but declare it.
Complete another declaration form for a change of major so that your present adviser can transfer you officially to a new program adviser.
Meet with your new program adviser and find out what courses you will need to take toward your new major.
Check with the registrar's office and make sure that your new major appears on your transcript. If it doesn't, find out if there are steps you need to take to ensure that it does.