home_page.dart

class HomePage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Stack(
        children: <Widget>[
          AirAsiaBar(height: 210.0),
          Positioned.fill(
            child: Padding(
              padding: EdgeInsets.only(
                  top: MediaQuery.of(context).padding.top + 40.0),
              child: new Column(
                children: <Widget>[
                  _buildButtonsRow(),
                  Container(), //TODO: Implement a card
                ],
              ),
            ),
          ),
        ],
      ),
    );
  }

  Widget _buildButtonsRow() {
    return Padding(
      padding: const EdgeInsets.all(8.0),
      child: Row(
        children: <Widget>[
          new RoundedButton(text: "ONE WAY"),
          new RoundedButton(text: "ROUND"),
          new RoundedButton(text: "MULTICITY", selected: true),
        ],
      ),
    );
  }
}

how change the content of tab's?

If you mean the labels of tabs, they're in the lines 30 to 32.
If you mean the children, in the latter parts of the post it's explained how to add content.
If you want to change content based on row, you could use StatefulWidget and for example IndexedStack. :)