How to change the input of the video capture card?

edit

Maybe your capture device maps each input with different device index... try enumerating multiple APIs and devices with this code... may be it works

Mat img;
VideoCapture cap;
string str;
vector<int> apiList({ CAP_DSHOW, CAP_MSMF, CAP_VFW });
for (int apiIdx = 0; apiIdx < apiList.size(); apiIdx++) 
{
    int api = apiList[apiIdx];
    for (int camIdx = 0; camIdx<10; camIdx++) 
    {
        cap.release();
        cap.open(camIdx + api);
        if (!cap.isOpened()) 
            continue;
        str = cv::format("API: %d device : %d", apiIdx, camIdx);
        cout << "Device found !! " << str << endl;
        cap >> img;
        if (img.empty()) {
            cout << "\t Unable to grab from " << str << endl;
            continue;
        }
        putText(img, str, Point(10, 20), FONT_HERSHEY_PLAIN,2, 
            Scalar(0, 255, 255),2);

        //a window for each device
        //imshow(str, img);  

        //a single window for all device
        imshow("Frame from current device", img);

        cv::waitKey(10);
    } // for devices
} //for APIs 
cout << endl << "Press Enter to terminate "; cin.get();