Sign in to your Apple ID account page once again. Replace your Apple ID with the email address you want to use. You’ll have to double-check the email address. On your iOS device, go to Settings > iCloud Sign in using your new Apple ID.
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To change apple id password, you need to go through the process of resetting it. This is done by going to Settings > General > Reset and Erase All Content and Settings. The process takes about an hour. Reference: how to apple id password.