To change the Tenant description which is shown at the top left of each page, or to change the Tenant id which is used to log in and is included in the web address go to System Admin and then to the Tenant Settings tab.
Click the pencil icon, change the description and / or id and click Save.
If you change only the Description you can refresh the browser page to see the change.
If you change the Id then you will need to login again using the new Tenant Id. One way to do this is to change the Id in the browser bar address and press Enter. That should take you to the new log in page.