I am working with jpeg files, when I look at the details of a jpeg file under windows (right-click->properties), under the Details tab there are entries for Title, Subject,
Here is some code to guide you.
ExifInterface newExif= new ExifInterface(filePath);
newExif.SetAttribute(ExifInterface.TagGpsLatitude, "teste lat");
newExif.SetAttribute(ExifInterface.TagGpsLatitudeRef, "teste lat ref");
newExif.SetAttribute(ExifInterface.TagGpsLongitude, "teste long");
newExif.SetAttribute(ExifInterface.TagGpsLongitudeRef, "teste long ref");
newExif.SaveAttributes();
You can use Android.Media.ExifInterface for getting/setting Exif Properties in Xamarin.Android.
http://developer.android.com/reference/android/media/ExifInterface.html