Let's explain this a different way.
To switch the default page.
1) Go to your Admin CP
2) On the side menu expand "Settings" and click on "Navigation Manager"
3) The Navigation Manager will look like this, though you probably have less tabs then I do. You will see "What's New" is listed as "Default."
4) To change the default find the drop down menu on the line of the tab you want to be default. In this example we will make "Forum" the new default. Choose "Set Default."
5) If necessary click "Go" after selecting Set Default- This will bring you to the final page-
6) Confirm your choice by clicking on "Save."
You will then be returned to the Navigation Manager and "Forum" will be the new default tab. So if anyone goes to
www.yoursite.com or
www.yoursite.com/index.php they will see the "Forum" page.