How to update my iPhone iOS?

The iPhone 6s Plus can update to the last update for iOS 15 (15.7.3) but no further. Settings > General > Software Update while connected to a charger, and Wi-Fi (not Cellular).

Your iPhone 6s Plus supports up to iOS 15.7.3. You can update it in Settings > General > Software Update. Make sure you have strong internet connection and at least 50% battery or 20% and plugged.