gistfile1.html

<dom-module id="x-app">
  <template>

    <template is="x-repeat" items="{{data.items}}">
      <div>{{item.name}}</div>
    </template>

    <button on-click="updateItem">update item</button>

  </template>
</dom-module>

<script>

  Polymer({
    is: 'x-app',
    properties: {
      data: Object
    },
    ready: function() {
      this.data = {
        items: [{name: 'foo'},{name: 'bar'},{name: 'zot'}]
      }
    },
    updateItem: function() {
      this.data.items[1] = {name: 'bar2'};
      this.notifyPath('data.items.1', this.data.items[1]);
    }
  });

</script>