Mat srcImg = new Mat(@"C:\Users\src.jpg");

OpenCvSharp.Point center = new OpenCvSharp.Point(srcImg.Width/2, srcImg.Height/2);
double angle = 180.0;

Mat affineMatrix = Cv2.GetRotationMatrix2D(center:center, angle:angle, scale:1.0);

Mat dstImg = new Mat();
Cv2.WarpAffine(src: srcImg, dst: dstImg, m: affineMatrix, dsize: dstImg.Size());

Cv2.ImShow("srcImg", srcImg);
Cv2.ImShow("dstImg", dstImg);