How can I update my iPhone 6s to latest iOS version?

Please note that an iPhone 6s cannot be used with iOS 16 or newer. The latest available and compatible version is iOS 15.8.3.

Update iOS on iPhone – Apple Support (UK)

I had an iPhone 6s. I replaced it when it was 6 years old. Your's is now 10 years old. That is nursing home old for a cell phone.

It will still continue to work, but as Jan mentioned, you are stuck at iOS 15.8.3