Android - Edit Action Bar Button text

I cannot find any way to edit the text of a button that shows on the action bar.

res/menu.main.xml
<item
    android:id="@+id/action_data"
    android:title="@string/action_date"
    app:showAsAction="withText|ifRoom"/>

And this what and how i'm trying to achive it:

MenuItem b = (MenuItem) findViewById(R.id.action_data);
b.setTitle(new SimpleDateFormat("dd-MM-yy").format(c));

Thanks for your help!