605 in 5.1

[BluHavik]

Sorry if it's already been asked. But I got the Onkyo HT-SP908 7.1 recently. Most of the Blu's I have are 5.1. How do I get the 605 to use the back speakers instead of the L n R surrounds for 5.1? I looked in the book, been no help. Note I know the speakers are hooked up right, when I try that mic thing I get a solid tone from all the speakers.

To the smart ass I'm sure will quip. This is my first surround foray. So yes, I am a moron. In this regard at least.