Are you sure it's an iPad Mini 2? The OP of this thread thought he had an iPad Mini 2 but it turned out he had an iPad Mini, which is not supported. Check the model number in the back to make sure. Mini 2 also needs to be at least iOS 8.
My iPad Mini 1 say: "This Update is not allowed for this device"...The Download takes s along time on the Apple I= Pad Mini 2. It will look like it is not downloading but give it about 10 minutes and you will see the( Dot Moving ) I tried for along time. My Daughter told me this