How do I delete a project?

Go to your projects listing (if you're already in Mockingbird you can hit Cmd/Ctrl-O or click on the "Account" button in the bottom toolbar and click Manage Projects.

Each project has a gear icon and if you click it you'll see the option to delete:

Click gear icon to access project settings, including the option to delete

Only the project owner has the option to delete a project, so if you see the gear icon but don't see the Delete option you aren't the owner. If you don't see the gear icon at all on a project, that means you haven't been granted access to edit the project settings.