You can use the ng-selected directive on the option elements. It takes expression that if truthy will set the selected property.
In this case:
<option ng-selected="data.unit == item.id"
ng-repeat="item in units"
ng-value="item.id">{{item.label}}</option>
Demo
angular.module("app",[]).controller("myCtrl",function($scope) {
$scope.units = [
{'id': 10, 'label': 'test1'},
{'id': 27, 'label': 'test2'},
{'id': 39, 'label': 'test3'},
]
$scope.data = {
'id': 1,
'unit': 27
}
});<script src="https://ajax.googleapis.com/ajax/libs/angularjs/1.2.23/angular.min.js"></script>
<div ng-app="app" ng-controller="myCtrl">
<select class="form-control" ng-change="unitChanged()" ng-model="data.unit">
<option ng-selected="data.unit == item.id" ng-repeat="item in units" ng-value="item.id">{{item.label}}</option>
</select>
</div>