Organizing and editing contact information within Microsoft Teams Free
Hello,
You put the Teams for free tag for this question, but it sounds like you are using Teams for business, can you please verify?
In addition, this guest user is not in your organization, right? Do you have admin access? If not, you need your Admin user to sign into Admin Center https://admin.microsoft.com > Users, and find the user and change it there. Changes can take up to 24 hours to reflect.