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Simply said, you need to work yourself into getting familiar with OpenCV like @berak suggested. Questions like this can be solved by simply performing 1 or 2 tutorials and getting the hang of everything. That said, the following pseudocode needs to be formed to your problem but it illustrates the desired functionality.

Mat image;
for(int i = 0; i < image.rows; i++){
   for(int j=0; j < image.cols; j++){
      //apply condition here
      if(image.at<type>(i,j) == threshold){
         image.at<type>(i,j) = 255;
      }
   }
}

Good luck!