Need latest update for my iPhone 6S plus

Hello ~Yes… the 6s is in fact compatible with is 15.1.1. Take a look here:

Identify your iPhone model - Apple Support

~Katana-San~

iOS 15.1.1

15.1.1 is not compatible with the 6s. How do you force an update to 14.8 instead?

This Apple Support document states that the 6S can indeed get iOS 15: iPhone models compatible with iOS 15.1 - Apple Support