set titles of items in my app's main menu?

I am trying to change the titles of some of the items in my Cocoa app's main menu. I have tried setting them both within IB and also programmatically from my app's applicationDidFinishLaunchingWithOptions: method. Either way, the title property of my NSMenuItem object does change. But none of my changes are reflected in the actual title of the item at the top of the screen when the app is running.

Can anyone explain what is going on? And how can I change this?

EDIT: The data structure is the default one that IB sets up:

NSApplication *app = [NSApplication sharedApplication];
NSMenu *mainMenu = [app mainMenu];
NSArray *itemArray = [mainMenu itemArray];
NSMenuItem *firstItem = [itemArray objectAtIndex: 0];
NSMenu *submenu = [firstItem submenu];

I have changed the title properties of both firstItem and subMenu to be my desired title. Yet the default one still shows.