Update multiple iPhones from iOS 11 to 12

[MaCheezm0]

I have to update over 100 iPhones at a location where wifi is slow. I don't have access to a mac. What are my options?

 

[winmaciek]

You can download ipsw once per phone model and restore them through iTunes.
Ipsw.me is a good place to get them (they link directly to Apple’s servers.)