You can only delete a brand if it does not contain any apps. So you must first delete all of the apps, one at a time, in the brand you wish to delete.
If you simply want to start fresh with a new brand name you can add a new brand for future Apps and leave your old Apps working in the old brand.
Once all of the apps have been deleted, a trash can icon will appear next to the brand and you can then press it to remove the brand.