Edit in JSFiddle

var APP = angular.module('APP', []);

APP.ApplicationCtrl = function ($scope) {
    
    $scope.name = '...';
    
    $scope.sayHelloWorld = function () {
        $scope.name = 'World';
    };
    
};
<div ng-app="APP" ng-controller="APP.ApplicationCtrl">
    <h1>Hello {{ name }}!</h1>
    
    <button ng-click="sayHelloWorld()">Say "Hello World"!</button>
</div>