i am going to tell you how:
when you download the file you want to connect your phone and open itunes(the latest version) once your done that go to to summary and press on check for update but when your press check for update at the same time press on option if your using a mac and if your using a windows laptop press on shift, when your done that your goign to select the file and then you will get a banner and it will tell you update and you will press yes or no and thats how you do it. and as always brrrrrrrrrpipto!!
the website:
https://ipsw.me/ or
http://www.ipodhacks142.com/how-to-downgrade-ios-11-beta-to-ios-10-3-3-10-3-2-on-iphone-ipod-touch-ipad/