Unable to update the email address of a AWS Account under AWS Organizations

You can update the email address on an Organizations-created account by logging in as the root user (to that account); see https://aws.amazon.com/premiumsupport/knowledge-center/change-email-address/ and https://docs.aws.amazon.com/awsaccountbilling/latest/aboutv2/manage-account-payment.html

However, before you can log in to an Organizations-created account you will need to reset the root password, because Organizations sets it to a randomised one and then discards it. This may be a problem if you cannot receive email at the address used for the account. See https://docs.aws.amazon.com/IAM/latest/UserGuide/troubleshoot-aws-sign-in.html#troubleshoot_general_lost-root-creds for some options.

You can update an account's email address by logging into that account's console. Even then you may find the old email address is still shown under Organizations - annoying, but doesn't have any impact.