how can I change defaults in Android?
Find the application which currently handles the action by default in your application list (Menu > Settings > Applications > Manage Applications).  Once you've found the app in question (note that the 'Manage Applications' list is almost, but not quite, in alphabetical order), tap it, and then tap the 'Clear defaults' button.

Then, next time you start an action that would previously have been handled by the app, you'll get the 'Which app should I use' popup.