If you are using Android you must have face this problem that some apps those are designed to work for same work e.g. handcent sms, go sms pro for messaging, or go contacts for your contacts. Always one dialog box come to ask from which apps you want to do you action or complete your action see the screen shot below:
If you check the tick “use by defaults for this action” next time you will not asked using this dialog box, but in case when you need to change your preferance you are in trouble how to change your default apps for certain action .
there is simple tricks for changing defaults using application manager, just go to settings and go application manager and in all apps choose choose you desired apps e.g. go-contacts and see below there is clear defaults icon tap this icon to remove defaults associated with this apps.
Now again you will be asked using dialog box for your defaults apps you can do this for any apps.