How do I change my message email

Are you trying to change your Apple ID or add another email to Messages?

To add another email to Messages, add the email to your Apple ID as a Reachable At email.

 Apple ID - Manage   

Apple ID email addresses. 

If you are trying to change your Apple ID, 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.