While performing actions on your new device for the first time you will most likely be asked to choose the applications which you will later on use to perform the same action. This should usually happen when you have two or more similar apps dedicated to do one thing (e.g. browsing the Internet - you have the Android's Internet, Chrome and Opera or Firefox if you wish to install them). When you open a link (for example from your email) you should be asked to choose which app use. It will be the same when you open an audio or movie file, text documents etc.
When decided which app should be used, you then should choose whether you would like this app to be set as default or just this one time. Having a default app may seem more convenient as you do not have to decide each time. However, what should you do when you have a default app set up but you wish to delete it and use a different appliaction? Follow the steps below to do it by yourself.
Go to Settings and choose Default Applications. You should be able to see the list of all of the apps which you have set as defaut. The only thing you have to do is click on "clear" and it will delete the unwanted setting.
Go to Settings and choose Applications Manager. Find the app (they should be listed alphabetically) and click on it. Scroll down to find "Clear defaults". When you click on it, the default app will be deleted.