If you've accidentally created a project under the wrong brand, you can simply edit it by clicking on "Edit details" button in the project overview.
Choose the required brand and click on "Update project".
All the tracked hours under the former brand will be edited and transfer to the newly selected brand.
All the assigned expenses or other external costs will be reassigned to the newly selected brand as well.
Note: You can change the brand in the project only in case there have been no invoices created already.