You won't be able to do that. If you try, you will be told it is unavailable or is already in use. It sounds like you are trying to change to an old email of yours or someone else's.
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You can't reuse an email address that was previously been associated with an Apple ID in any way until an uncertain period has passed--sometimes reputed to be 30 day and sometimes 1 year. Also, I have seen that you never can use such an email address again. The last two times I saw a wait period claimed it was said to be “up to 30 days.” You can contact Apple support to check out the wait period.
You can get Apple telephone support for help using this link https://getsupport.apple.com/?caller=kbase&PGF=PGF63005&category_id=SC0245&symptom_id=23362 Be patient sometimes Apple support is closed to immediate phone support.
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And, see Turn off Activation Lock https://support.apple.com/en-us/HT201441. All possibilities related to iPhones, iPads, and iPods are covered here except the case of inheritance.
If you are the original owner: If you have iOS 11 or later and your account is protected with two-factor authentication, use get help with a forgotten passcode or disabled device to recover your passcode. Otherwise, use forgot your Apple ID or your Apple ID password to recover the needed credential(s). If you can’t recover the needed credential(s), contact Apple Support and present your original purchase receipt for your device from an authorized Apple seller with the device’s serial number on it. You’ll need a photo ID too.If you can do neither, then the device is useless to you.
If you acquired your device second hand, you usually will need to obtain the needed password from the person that last set it up. If you can’t obtain it, in the rare case your device came activation locked from an authorized Apple seller, then contact Apple Support and present your original purchase receipt for your device with the device’s serial number on it. You’ll need a photo ID too. If you can do neither, then the device is useless to you.
In both cases, you can use the suggested support options below. Alternatively, if an Apple Store is open in your area, then you can go there to deal with your problem. In this case, you can make a genius-bar appointment (https://www.apple.com/retail/geniusbar/) if you want.
Get a call from Apple support to help solve your problem using this
link https://getsupport.apple.com/?caller=kbase&PGF=PGF63005&category_id=SC0245&symptom_id=23362
Apple ID Support https://support.apple.com/apple-id
Apple ID account security for country-specific andregion-specific help https://support.apple.com/en-us/HT204169
Get Support: We’re here to help https://getsupport.apple.com/. Go through the options to find the best support option.
