public string Title {get; set;}

{
  // Note: the following should be embedded in a QueuedTask.Run() statement
  {
    // GetRenderer from Layer (assumes it is a unique value renderer)
    if (featureLayer.GetRenderer() is not CIMUniqueValueRenderer uvRenderer)
    {
      // not a unique value renderer, leave
      return;
    }
    //layer has STATE_NAME field if using the community sample Data\Admin\AdminSample.aprx
    string expression = "if ($view.scale > 21000000) { return $feature.STATE_NAME } else { return 'All' }";
    CIMExpressionInfo updatedExpressionInfo = new()
    {
      Expression = expression,
      Title = "Custom" // can be any string used for UI purpose.
    };
    //set the renderer's expression
    uvRenderer.ValueExpressionInfo = updatedExpressionInfo;

    //SetRenderer on Layer
    featureLayer.SetRenderer(uvRenderer);
  }
}