Android : how to change button text or drawable in a notification with custom layout?

I am implementing a notification for when a file is being downloaded to show the progress. also I want the user to be able to pause/resume downloading directly from the notification using a button (similar to what you see in an audio player notification for play/pause). I am familiar with setOnClickPendingIntent. what I can't figure is how to change button's text toggling between "pause" and "play" or drawable on user click.