I think you need a copy of an old signed ipsw file (you may already have one on your computer or you can find these on the internet) Then you can either put your Apple device in recovery mode or preferably open iTunes, choose your device, then hold down shift and click restore and then should be...
I think on older devices such as iPhone 4s and early iPads it worth seeing what people feedback before updating to make sure it does not slow down your device although I guess you can downgrade again but just a lot of hassle