apple id can not be changed

Are you trying to do an email address change on your existing Apple ID? Are you using the steps in the article below? If so and it isn't working, what error message(s) are you getting?

Note the restrictions in the article.

Apple ID Name Change.        

 What to do after you change your Apple ID or password - Apple Support    I suggest doing this before you change your Apple ID or password.

If the above isn't what you are trying to do, please try restating your question and explain exactly what you are trying to do. Include whatever problems you are having and any error messages.