In the Reports > Builder screen select the report template you wish to use.
In the settings under Row Data you will need to select ADVANCED where you can choose to toggle between Accounts or Business Units for the Display as Rows settings.
The Display as Rows groups are set by the Level 1 Headers of the Account Tree that you select.
You can use a mixture of Accounts and Business Units in the same report if required, as an example you could set:
Income Header = Business Units as Rows
Cost of Sales Header = Business Units as Rows
Expense Header = Account Rows
Other Income = Account Rows
Other Expenses = Account Rows