How to change default landing tab for facebook page ?

To the best of my knowledge,
There is no way you can achieve this by writing a program.

Though, you can do it in two different ways.

1) Let user add you facebook application to his page as mentioned in previous post.

Now, go to https://www.facebook.com/pages/edit/?id=<page id>. (or go to page and click on edit page).

Go to "Default Landing Tab".

Choose the application / tab you want to set as default.

2) Many times, when you are developing third party app which users are going to install and configure like http://apps.facebook.com/involver_hoplvaex/?ref=ts . After the user installs it and configures it, you would like user to go to direct your app tab to see the preview rather than going to its wall. You can achieve this by adding application id at the end of the URL.

i.e. https://www.facebook.com/pages/<page name>/<page id>?v=app_<app id>

How can facebook page owners install third party facebook app ?

There are two ways for installing any app on facebook page. 


 1. goto https://www.facebook.com/apps/application.php?id="" click on add to my page.

But, what if you want user to install your app just by clicking on the install button available on either your app page or on any website page? 


When user clicks on install button, redirect page to : 
http://www.facebook.com/add.php?api_key="<api_key>"&pages 


So, the user will be asked if he wants to add application to his pages (in case he owns a single page). 
 If the user owns multiple pages, he will be asked to choose pages for the app to be installed on.