To change your Craft account email, go to Preferences > My Account > Change email
Unable to Change Email / Email already in Use
Please note you can only change your account email to an email address that it's not currently a Craft account.
However, once a Craft account is deleted, the email associated with it will become available again for sign-up. Therefore, you may delete the account whose e-mail you want to use in Profile > Settings > My Account > Delete Account and afterward make the email change in the remaining account in Profile > Settings > My Account > Change email.
Important: Deleting an account will permanently delete all documents. Make sure to back up any important data before deletion.