public Envelope Location {get; set;}

public Task UpdateBookmarkExtentAsync(Bookmark bookmark, ArcGIS.Core.Geometry.Envelope envelope)
{
  return QueuedTask.Run(() =>
  {
    //Get the bookmark's definition
    var bookmarkDef = bookmark.GetDefinition();

    //Modify the bookmark's location
    bookmarkDef.Location = envelope;

    //Clear the camera as it is no longer valid.
    bookmarkDef.Camera = null;

    //Set the bookmark definition
    bookmark.SetDefinition(bookmarkDef);
  });
}