The average student may change his originally declared major three to five times during his undergraduate career. In most universities, you can change your major by scheduling an appointment with your adviser or filling out the paperwork in the relevant department office. Some universities require you to write a letter, in which case a brief explanation of your rationale will do.
Address the letter to your adviser. If you don't know who your adviser is, call your academic department and ask for her name. Begin the first paragraph by explaining right away that you would like to change your major. Give the full name of the degree program you were in and give the new program. If you will switch your minor as well, provide that information too. For example, write "I would like to change my degree plan from a B.S. in Chemistry with a minor in Biology to a B.S. in Biology with a minor in Psychology."
Explain the reason that you are changing your major in one paragraph. For instance, "I am interested in changing from a chemistry major to a biology major because I have decided that I would like to pursue a degree in medicine after my undergraduate degree, and a degree in biology is more applicable to medicine. I feel that a minor in psychology will help me understand the trauma that patients with severe injuries go through, which will help me become a better physician."
If your new major is in the same academic department, such as moving from a B.A. in Literature to a B.A. in Technical Writing within the English Department, you will probably retain the same adviser. Ask your adviser (in the letter) for an appointment to sign any necessary forms and to discuss the specifics of your new degree plan. Changing your major might change your graduation requirements. If your change in major places you in a different academic department, contact the new department for advising.