You can change a team name anytime. In fact there are two potential names, one for internal view and one for external view.
You must have administrator permissions. Click on 'Admin' > 'Name & Theme'. You will see two inputs at the top of that page called 'Name' and 'Public name'.
The name used within the web app cannot be edited anymore and groups will retain the name from when they are created. Public names can still be changed.
In the same page you will see 'theme'. Set the colours of the web app theme for your whole team. Click here for more about themes.
Enter your new name and then press Save at the foot of the page. Name is used within the web app itself, while the public name is used for customer-facing situations, such as emails and the signing pages.