How to open IPhone 6S with knife without breaking screen and replace battery?

I knife is almost guaranteed to break a screen. You need to apply heat to loosen the adhesive and something flexible like a guitar pick or spudger.

https://www.amazon.com/Opening-Kit-Plastic-Spudger-Computer/dp/B0D14WYPDQ

The only part of a Swiss Army knife that might be useful here is the plastic toothpick, and that’s not the best shape.

A metal blade prying on glass? No way.

The Apple battery replacement cost for iPhone 6s is US$69.

Buying a third-party battery and the necessary screwdrivers will be a chunk of that $69 cost, too. (The folks at I Fix It have some good quality tools available for this, as the cheap ones I’ve bought have all rounded out.)