"expander_group.xml" goes in \res\drawable\
<selector xmlns:android="http://schemas.android.com/apk/res/android">
    <item
        android:state_expanded="true"
        android:drawable="@drawable/arrowdown" />
    <item
        android:drawable="@drawable/arrowright" />
</selector>
-----------------------------------------------------
in your layout:

    <ExpandableListView
        android:id="@+id/listView"
        android:layout_width="wrap_content"
	android:layout_height="wrap_content"
	android:fadingEdge="none"
	android:scrollingCache="false"
	android:groupIndicator="@drawable/expander_group"
	>
    </ExpandableListView>