So, first you have to think of the reasons, and write them down.
You reasons will fall roughly into two groups: reasons why you want to quit your current course, and reasons why you want to do the different course.
You should try to make it seem that you really want to do the new course. For example, you don't say you're bored and want to have a shot at something else for a semester or two.
Anyhow, only you know the details. If you write that much and post it, someone can help you refine it.