UITabBarController
has a property called viewControllers
. It's an array that contains all the view controllers that the tab-bar controller can represent. By accessing the tab-bar controller from one view controller, you can access that array, and then you can then access the desired view controller whose text outlet you can change when the button from the first view controller is pressed.
I'm not entirely sure how to reference the UITabBarController from one of its child view controllers. It may be a preexisting property that is set during creation. If that's not the case. You can give all the view controllers of the tab-bar controller a reference to the parent manually during creation.
Let me know if you need further help.