Grab your Android device and navigate to the Settings menu (either by tapping the physical Menu Button and selecting Settings or by opening your application list and selecting Settings as seen in the first panel of the screenshot above). Navigate to Applications and then to Manage applications.
Once in the Manage applications sub-menu tap on All to list all the applications installed on your phone. Scroll down until you see the application you wish to change. For this example we will be changing our home screen manager from LauncherPro to ADW.Launcher to demonstrate how to change a default system application.