01– Click edit page (upper right corner)

02- Halfway down you should see the field “Default Landing Tab”

03– Select your default landing page from drop down

(Note: As you create more tabs, these tabs pages will show under this list as well)

Previous methods used by Facebook have changed since its upgrade

Related Queries:
How to make a custom Tab page show up on my Fanpage?
Method for selecting custom tabs in Facebook to appear upon visit
How do I change my default Facebook page?
I created a custom tab, how do I get that to show up as my default?