setSupportActionBar(toolbar);
    getSupportActionBar().setDisplayHomeAsUpEnabled(true);
    getSupportActionBar().setHomeButtonEnabled(true);

<string name="main_activity_name”>Main Activity</string>

<activity
    android:name="MainActivity"
    android:label="@string/main_activity_name” >
</activity>

MainActivity.mName = “new name”;

public static String mName = “old name”;

getSupportActionBar().setTitle(mName);