public CIMTextGraphic TextGraphic {get;}

// Modifies the text graphic of a selected annotation feature in the specified annotation layer.
await QueuedTask.Run(() =>
{
  var selection = annotationLayer.GetSelection();
  if (selection.GetCount() == 0)
    return;
  // use the first selected feature 
  var insp = new Inspector();
  insp.Load(annotationLayer, 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
  {
    Name = "modify symbol"
  };
  op.Modify(insp);
  if (!op.IsEmpty)
  {
    bool result = op.Execute(); //Execute and ExecuteAsync will return true if the operation was successful and false if not
  }
});