How to set orientation property of an image before save?

Question

foreach (var prop in img.PropertyItems)

{

      if (prop.Id == 0x0112) //value of EXIF

      {

         var orientation = img.GetPropertyItem(prop.Id).Value[0];

         switch (orientation)

         {

            case 1:

            // No rotation required.

            break;

            case 2:

            img.RotateFlip(RotateFlipType.RotateNoneFlipX);

            break;

            case 3:

            img.RotateFlip(RotateFlipType.Rotate180FlipNone);

 

 

I have used above code to detect whether image orietation is changed from original. Suppose I have get orientation value as "3" and I want to set it to normal that is "1", so how I could replace that orientation property value 3 by 1...??