Update - February 4, 2019 At the time of this writing as far as I am aware you can rename a Free Microsoft Teams organization.
Steps to rename your Microsoft Teams (free) Organization: Identify your onmicrosoft.com tenant id. Login to http://account.activedirectory.windowsazure.com/ and click your profile in the top right, note the "firstname.lastname@example.org" email address. Login to portal.office.com using the email@example.com account, you may need to select "Can't access your account" link when selecting "Use another account" from the login window/popup. You should be able to send a reset to your linked account (i.e. outlook.com). Go to the Administration panel, then select Billing. In the Billing dropdown, select 'Subscriptions' then click on the Microsoft Teams (free) subscription. Click on your organization name at the top of the screen, and edit it to the new name you want for your organization. NOTE: The changes may take an hour or so to be reflected in your desktop app, web, or mobile application.
Additionally, I would not attempt to have the service deactivated as indicated by @Christopher Hoard : Delete Free Teams Organization - if your goal is to rename your organization. After having tried to follow this Support article originally, my outlook account was unable to recreate a Free Microsoft Teams subscription.