public string DefaultToolGUID {get; set;}

public Task ChangeTemplateDefaultToolAsync(ArcGIS.Desktop.Mapping.FeatureLayer flayer,
                  string toolContentGUID, string templateName)
{
  return ArcGIS.Desktop.Framework.Threading.Tasks.QueuedTask.Run(() =>
  {

    // retrieve the edit template form the layer by name
    var template = flayer?.GetTemplate(templateName) as ArcGIS.Desktop.Editing.Templates.EditingTemplate;
    // get the definition of the layer
    var layerDef = flayer?.GetDefinition() as ArcGIS.Core.CIM.CIMFeatureLayer;
    if ((template == null) || (layerDef == null))
      return;

    if (template.DefaultToolID != this.ID)
    {
      bool updateLayerDef = false;
      if (layerDef.AutoGenerateFeatureTemplates)
      {
        layerDef.AutoGenerateFeatureTemplates = false;
        updateLayerDef = true;
      }

      // retrieve the CIM edit template definition
      var templateDef = template.GetDefinition();

      // assign the GUID from the tool DAML definition, for example
      // <tool id="TestConstructionTool_SampleSDKTool" categoryRefID="esri_editing_construction_polyline" ….>
      //   <tooltip heading="">Tooltip text<disabledText /></tooltip>
      //   <content guid="e58239b3-9c69-49e5-ad4d-bb2ba29ff3ea" />
      // </tool>
      // then the toolContentGUID would be "e58239b3-9c69-49e5-ad4d-bb2ba29ff3ea"

      //At 2.x -
      //templateDef.ToolProgID = toolContentGUID;
      templateDef.DefaultToolGUID = toolContentGUID;

      // set the definition back to 
      template.SetDefinition(templateDef);

      // update the layer definition too
      if (updateLayerDef)
        flayer.SetDefinition(layerDef);
    }
  });
}