How do I downgrade Xbox 360 dashboard?
There is no easy way of downgrading Xbox 360 Dashboard, since there are physical measures put in place to prevent just that (CPU e-Fuses blow when upgraded to prevent downgrade).
I heard there is a possibility of downgrading certain Dashboard versions by a timing attack, but that requires advanced electronics knowledge (and equipment) and definetly voids your warranty.
Basically what you have to do is JTAG your Xbox, then it will work. I'm no expert so double check on YouTube or something. I'm pretty sure, though!