in that way, you have not declared a button, but only an action called 'button'.
in your .h file you should do:
@interface yourViewController : UIViewController {
UIButton *button;
}
and for change title of segment control you can do:
[yourSegmentControl setTitle:@"Button!" forSegmentAtIndex:0]; //0 is the first
if you want change a title of an UIButton:
[button setTitle:@"Title" forState: UIControlStateNormal];