Cannot change apple id

Are you trying to change the Mail address of your Apple ID, as explained in this article?

Did you recently create this Apple ID?

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If you can't change your Apple ID

If you try to change your Apple ID to an @icloud.com email address that was created within the last 30 days, you will be asked to try again later.

If you see a message that the email address you want to use isn't available or is already in use, check that you or a family member aren't already using that email address with a different Apple ID. If a family member is using that email address, remove it from their account then try to use it. 

In some countries or regions, you can use your phone number as your Apple ID. But if your Apple ID is an email address, you can't change it to a phone number.

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copied from: Change your Apple ID - Apple Support

Any error messages shown?

Why do you want to? What happens when you try?

then we can help with the relevant advice. Most people only want to change the Password.

or go to https://appleid.apple.com/account/manage

but you might be safer if you reach out to Apple Support directly. You should be able to reach them here

Contact - Official Apple Support

or phone numbers

Contact Apple for support and service - Apple Support