public AnnotationProperties GetAnnotationProperties()

await QueuedTask.Run(() =>
{
  //annoLayer is ~your~ Annotation layer...

  // use the inspector methodology
  var insp = new Inspector(true);
  insp.Load(annoLayer, oid);

  // get the annotation properties
  AnnotationProperties annoProperties = insp.GetAnnotationProperties();
  // set the attribute
  annoProperties.TextString = "Hello World";
  // assign the annotation proeprties back to the inspector
  insp.SetAnnotationProperties(annoProperties);

  //create and execute the edit operation
  EditOperation op = new EditOperation();
  op.Name = "Update annotation";
  op.Modify(insp);
  op.Execute();
});

await QueuedTask.Run(() =>
{
  //Don't use 'Shape'....Shape is the bounding box of the annotation text. This is NOT what you want...
  //
  //var insp = new Inspector();
  //insp.Load(annoLayer, oid);
  //var shape = insp["SHAPE"] as Polygon;
  //...wrong shape...

  //Instead, we must use the AnnotationProperties

  //annoLayer is ~your~ Annotation layer
  var insp = new Inspector(true);
  insp.Load(annoLayer, oid);

  AnnotationProperties annoProperties = insp.GetAnnotationProperties();
  var shape = annoProperties.Shape;
  if (shape.GeometryType != GeometryType.GeometryBag)
  {
    var newGeometry = GeometryEngine.Instance.Move(shape, 10, 10);
    annoProperties.Shape = newGeometry;
    insp.SetAnnotationProperties(annoProperties);

    EditOperation op = new EditOperation();
    op.Name = "Change annotation angle";
    op.Modify(insp);
    op.Execute();
  }
});

await QueuedTask.Run(() =>
{

  var selection = annoLayer.GetSelection();
  if (selection.GetCount() == 0)
    return;

  // use the first selelcted feature 
  var insp = new Inspector(true);
  insp.Load(annoLayer, selection.GetObjectIDs().FirstOrDefault());

  // getAnnoProperties should return null if not an annotation feature
  AnnotationProperties annoProperties = insp.GetAnnotationProperties();
  // get the textGraphic
  CIMTextGraphic textGraphic = annoProperties.TextGraphic;

  // change text
  textGraphic.Text = "Hello world";

  // set x,y offset via the symbol
  var symbol = textGraphic.Symbol.Symbol;
  var textSymbol = symbol as CIMTextSymbol;
  textSymbol.OffsetX = 2;
  textSymbol.OffsetY = 3;

  textSymbol.HorizontalAlignment = HorizontalAlignment.Center;

  // load the updated textGraphic
  annoProperties.LoadFromTextGraphic(textGraphic);
  // assign the annotation properties back 
  insp.SetAnnotationProperties(annoProperties);

  EditOperation op = new EditOperation();
  op.Name = "modify symbol";
  op.Modify(insp);
  bool result = op.Execute();
});