Take a look at the header AVCaptureSession.h. There is a definition for an enum called AVCaptureVideoOrientation that defines various video orientations. On the AVCaptureConnection object there is a property called videoOrientation that is a AVCaptureVideoOrientation. You should be able to set this to change the orientation of the video. You probably want AVCaptureVideoOrientationLandscapeRight or AVCaptureVideoOrientationLandscapeLeft.
You can find the AVCaptureConnections for the session by looking at the outputs for the session. The outputs have a connections property that is an array of connections for that output.