As an organization owner, you can do the following to unlink an external identity:

• Navigate to https://github.com/orgs/[organization_name]/people/[username]/sso
• Under the Linked SSO identity tab, click on the Revoke button
• Click the Revoke external identity button to confirm

Replace [organization_name] with the name of your organization and [username] with the username that has the idenity linked currently, which you want to unlink (that is, your user’s old username, not their current one).