There are two different ways to downgrade a N9 that I know of, but neither is especially easy 1.) There's a certain guy in Tampere, Finlad who can do it for you, The downside is you have to travel there and know who to ask for
2.) If you have (or can get hold of) another N9 which is on lower software leve than your device there is a way to use ubiboot to duplicate the FS of the pristine device to your device