If you have a navigation bar across the top, then you can set the title like this:
self.title = @"this is my title";
If your title is fixed and doesn't change, then just add that line in your viewDidLoad method or where ever you would setup items in your view.
If you are trying to set the title programmatically, then it depends on which view controller you are in when you try to set the title. In the example above, it will set it for the view controller you are in.
If you want to set the opposite view controller navigation bar, you have a couple of ways to choose from depending on when you want to update it.
Generally though you would reference the opposite view controller directly using a delegate or indirectly using the splitViewController.viewControllers array.
it sounds like you already have a reference to the detail view controller. If so and you want to change its title from the master you can do so like this:
[self.detailViewController setTitle:@"New Title"];
hope that helps,
be well