Change the actionbar homeAsUpIndicator Programmatically

This is what i did to acheive the behavior. I inherited the base theme and created a new theme to use it as a theme for the specific activity.

<style name="CustomActivityTheme" parent="AppTheme">
    <item name="android:homeAsUpIndicator">@drawable/custom_home_as_up_icon</item>
</style>

and in the android manifest i made the activity theme as the above.

<activity
        android:name="com.example.CustomActivity"
        android:theme="@style/CustomActivityTheme" >
</activity>

works great. Will update again when i check on all devices I have. Thanks @faylon for pointing in the right direction

The question was to change dynamically the Up Home Indicator, although this answer was accepting and it is about Themes and Styles. I found a way to do this programmatically, according to Adneal's answer which gives me the clue and specially the right way to do. I used the below snippet code and it works well on (tested) devices with APIs mentioned here.

For lower APIs, I use R.id.up which is not available on higher API. That's why, I retrieve this id by a little workaround which is getting the parent of home button (android.R.id.home) and its first child (android.R.id.up):

if(Build.VERSION.SDK_INT >= Build.VERSION_CODES.ICE_CREAM_SANDWICH) {
    // get the parent view of home (app icon) imageview
    ViewGroup home = (ViewGroup) findViewById(android.R.id.home).getParent();
    // get the first child (up imageview)
    ( (ImageView) home.getChildAt(0) )
        // change the icon according to your needs
        .setImageResource(R.drawable.custom_icon_up));
} else {
    // get the up imageview directly with R.id.up
    ( (ImageView) findViewById(R.id.up) )
        .setImageResource(R.drawable.custom_icon_up));
} 

Note: If you don't use the SDK condition, you will get some NullPointerException.

API 18 has new methods ActionBar.setHomeAsUpIndicator() - unfortunately these aren't supported in the support library at this moment

http://developer.android.com/reference/android/app/ActionBar.html#setHomeAsUpIndicator(android.graphics.drawable.Drawable)

edit: these are now supported by the support library http://developer.android.com/reference/android/support/v7/app/ActionBar.html#setHomeAsUpIndicator(android.graphics.drawable.Drawable)