How to customize the back button on ActionBar

So you can change it programmatically easily by using homeAsUpIndicator() function that added in android API level 18 and upper.

ActionBar().setHomeAsUpIndicator(R.drawable.ic_yourindicator);

If you use support library

getSupportActionBar().setHomeAsUpIndicator(R.drawable.ic_yourindicator);

The "up" affordance indicator is provided by a drawable specified in the homeAsUpIndicator attribute of the theme. To override it with your own custom version it would be something like this:

<style name="Theme.MyFancyTheme" parent="android:Theme.Holo">
    <item name="android:homeAsUpIndicator">@drawable/my_fancy_up_indicator</item>
</style>

If you are supporting pre-3.0 with your application be sure you put this version of the custom theme in values-v11 or similar.