Vec3b is a vector of three elements of uint8 type.
cv::Mat somepicture; // that is hopefully CV_8UC3, the usual format
// use VideoReader to read frames into `somepicture`
Vec3b& pixel = somepicture.at<Vec3b>(i,j); // get a pixel (reference) from i-th row (y), j-th column (x)
// pixel is blue, pixel is green, pixel is red
pixel = 255 - pixel; // invert the blue value (example)
PLEASE work through the tutorials. this is the very basics of how to work with OpenCV.
there may be high level methods that might do what you need faster (recommended). if there aren’t, you’ll have to do your own looping over the pixels, or possibly use OpenCV’s