how do i change my apple id on my iphone

Two ways to interpret your question:

Sign out of iCloud on your iPhone, iPad, iPod touch, Apple TV, or Mac --> https://support.apple.com/HT208242

If another Apple ID is signed in you may need to enter the password for that ID to sign out of that ID first.

Click on this link to learn how to 'Sign in with your Apple ID' --> https://support.apple.com/HT204053

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Change your Apple ID - https://support.apple.com/HT202667 - "Follow these steps to change the email address that you use as your Apple ID."

Two possibilities I can think of:

Contact Apple and see if they can sort out the ID email issue:

Contact Apple for help with Apple ID account security - https://support.apple.com/HT204169 "This article provides country-specific Apple Support contact information for customers seeking help with their Apple ID password or other security-related issues."

OR

Apple can completely reset a device if a person can provide sufficient evidence of valid ownership. Do this online at: Activation Lock support request - https://al-support.apple.com/#/additional-support Apple says, "Proof of ownership documentation is required," but does not state what currently constitutes proof. Prior to this new online service Apple typically required the original receipt issued by Apple or an Apple Authorized Reseller and documented provenance (so typically it would be the original owner who does this, or a descendant with legal proof of ownership).