When it comes time to switch from using one application to another on  your Android device it isn’t immediately clear how to do so. Follow  along as we walk you through swapping the default application for any  Android task.
Initially changing the default application in Android is a snap.  After you install the new application (new web browser, new messaging  tool, new whatever) Android prompts you to pick which application (the  new or the old) you wish to use for that task the first time you attempt  to open a web page, check your text message, or otherwise trigger the  event. Easy! What about when it comes time to uninstall the app or just  change back to your old app? There’s no helpful pop-up dialog box for  that. Read on as we show you how to swap out any default application for  any other with a minimum of fuss.
Changing the Default Application
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.
Inside the Application info menu for the application scroll down and tap Clear defaults.  Once you have cleared the defaults you can then force a new default  selection by triggering the action that the application would handle. In  our case we’re swapping out the home screen manager so all we have to  do to trigger the event is tap the physical Home button on our phone. Android then prompts us with a Complete action using dialog box. Here we can select either ADW.Launcher or LauncherPro. Note the Use by default for this action checkbox. If you don’t want to be hassled to pick in the future, commit to your change here.
Potential Pitfalls and Workarounds
If you’re having trouble forcing the Complete action using  dialog box to appear (especially for the handling of non-default file  types) make sure you haven’t uninstalled the prior default application.  Android sometimes gets hung up when attempting to change the default  application for a file type or action away from the prior selection if  the prior selection has been uninstalled.
For example, if you installed a video application and associated it  with a bunch of video file types and removed the application before  switching the default application you might run into problems. In this  case the best solution is to install the old application again, change  the default from within the Application info screen for  that app, and remove it once you’ve successfully used the new  application. It’s a hassle, we know, but now that you know how to change  the task associations you shouldn’t be stuck in that situation again.