You can change your Apple ID Mail address by following the steps in this related article:
Change your Apple ID - Apple Support
Also have a look at this part mentioned in that article:
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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.
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copied from: Change your Apple ID - Apple Support
And, there are a few things to do after changing it:
What to do after you change your Apple ID or password - Apple Support