How to

internal class ActivateMapFrameBtn: Button {

 protected override void OnClick()
 {
  //1. must be on the GUI - dont switch to QueuedTask

  //2. Layout view must be active!
  if (LayoutView.Active == null)
    return; 

  //3. LayoutView.Active.ActiveMapFrame is the frame that will
  //   be activated
  var map_frameName = "Map Frame";
  if (LayoutView.Active.ActiveMapFrame == null ||
     LayoutView.Active.ActiveMapFrame.Name != map_frameName) {

     //simply select it to make it the active one
     var selFrame = LayoutView.Active.Layout.GetElementsAsFlattenedList()
        .OfType<MapFrame>().FirstOrDefault(mf => mf.Name == map_frameName);
     if (selFrame == null)
       return; //not found

     LayoutView.Active.Layout.SelectElement(selFrame);
   }

   //4. Does the ActiveMapFrame have a valid map?
   if (LayoutView.Active.ActiveMapFrame?.Map == null)
     return; //invalid map frame - probably empty

   //esri_layouts_activateMap - activate
   //esri_layouts_deactivateMap - deactivate

   //Do the activation
   var plugin = 
       FrameworkApplication.GetPlugInWrapper("esri_layouts_activateMap") 
                                         as System.Windows.Input.ICommand;
   if (plugin.CanExecute(null))
     plugin.Execute(null);
 }
}