apple id

That may mean that it's the secondary email for your Apple ID. If that's the case, you'll need to change the secondary email to a third email address (get a free one from Google or Yahoo). Once that change is complete, you should be able to change the primary address to the one you want.

The other possibility is that the email address is associated with a different Apple ID, that you actually have more than one.

Here is how you can change your Apple IDs email:

Change your Apple ID - Apple Support

I hope this helps!