Find my iPhone app won't install on iPhone 6s
I'm using iOS 10 Developer Beta 7. Find my iPhone is already installed. Just out of curiosity, I went to the App Store, and did a search for the app. Interestingly, the little "cloud" icon would seem to indicate that, although I have downloaded it in the past, it is NOT installed. This is not actually the case - it IS installed. 



Try going to your main page and swiping down - this will enable the search feature. Then search for "Find iPhone." Just in case it's hidden in a folder, this will tell you if it is already installed.