If you, for example, install multiple media player applications on your Android device, you’ll find that Android will give you the choice of which application to launch by default when you open a media file or document. However, you may later want to change this default setting.
To do so, go to Settings > Applications > Manage applications and then press the "All" tab. Choose the application that you previously selected as default, and scroll to the section "Launch by default" and press the "Clear defaults" button. Next time you open that particular type of file in the future, Android will prompt you again to choose a new default app.