How do I update my iPhone 6s Plus software?

Tap Settings > General > Software Update to see if an update is available for your phone.

The iPhone 6s Plus cannot update to iOS 16 or iOS 17, if that might be what you are looking for.

Your iPhone 6s Plus cannot run iOS 16 or 17.

From time to time, Apple may release a bug fix or security patch update to iOS 15. As of today, the latest release of iOS 15 is iOS 15.8.2. If you run into problems updating your iPhone to the latest release of iOS 15, see the User Tip linked below.

About iOS 15 Updates - Apple Support

Updating iOS: common issues and solutions - Apple Community

The iPhone 6S+, released by Apple 8½ years ago, cannot be updated beyond iOS 15.X.