Answer:
If you are refering to "Habbo Beta", then as of this moment, all you have to do is click "Enter Hotel". Habbo was testing "Habbo Beta" in 2009 [on US atleast] and when you logged in you could choose to try Habbo Beta, or continue using "Old Habbo". Habbo switched, I believe, all of the hotels [every language] to Beta by default in late 2009. As of this moment, you cannot change back to Old Habbo.