This is old trick, but still valid even in the latest version of Dynamics 365.
By default, your root business unit name is the same as the CRM instance name.
If you want to change it, you can go to the Security Settings > Business unit.
Select the root Business unit, open it. You will see that the Parent business unit is mandatory and you are not able to save your changes.
You need to go to the Customization > Business units > Fields and change the parentbusinessunitid to Optional.
Then publish the customizations and go to edit the root business unit.
You will be able to change the name of the root business unit and save it.
Stay tuned for the next tip!