NEVER downgrade via restoring a device without using DFU mode. iclarified has a tutorial (http://www.iclarified.com/31004/how-to-downgrade-from-ios-7-beta-to-ios-6)
But I'm not downgrading.
I have a jailbroken iPhone 5 on iOS 6.1 and I want to install iOS 7 b1 on it instead.
So all I do is restore it to iOS 6.1 (so that the jailbreak is removed) and install iOS 7 afterward?