Update parent scope variable in AngularJS

You need to use an object (not a primitive) in the parent scope and then you will be able to update it directly from the child scope

Parent:

app.controller('ctrlParent',function($scope){
    $scope.parentprimitive = "someprimitive";
    $scope.parentobj = {};
    $scope.parentobj.parentproperty = "someproperty";
});

Child:

app.controller('ctrlChild',function($scope){
    $scope.parentprimitive = "this will NOT modify the parent"; //new child scope variable
    $scope.parentobj.parentproperty = "this WILL modify the parent";
});

Working demo: http://jsfiddle.net/sh0ber/xxNxj/

See What are the nuances of scope prototypal / prototypical inheritance in AngularJS?

There is one more way to do this task and to not use the $scope.$parent variable.

Just prepare a method for changing the value in parent scope and use it in child one. Like this:

app.controller('ctrlParent',function($scope) {
  $scope.simpleValue = 'x';
  $scope.changeSimpleValue = function(newVal) {
    $scope.simpleValue = newVal;
  };
});

app.controller('ctrlChild',function($scope){
    $scope.changeSimpleValue('y');
});

It also works and give you more control over the value changes.

You can then also call the method even in HTML like: <a ng-click="changeSimpleValue('y')" href="#">click me!</a>.