iCloud pop up every 30 seconds asking me to sign in. It is using an old email address and I have changed my email address at Apple. It won't let me change the email address in the sign-on window.

Every 30 seconds, a screen pops up (just started this morning). iCloud wants me to sign in. However, it is using an email address that is no longer associated with my account. It will let me put in my password but will not allow me to change the email address.

Please advise me on how to fix this or, at least, stop the darn window from popping up every 30 seconds.

Thank you.