How do I upgrade to iOS 17?

If your signature is correct and you own an iPhone 6s, you cannot update to iOS 17. iOS 15.7.9 is the highest it can run. If you want iOS 17, you'll need to retire your 9 year old iPhone and purchase a newer, more modern iPhone.

See: How do I update to a 17 iOS - Apple Community

First you would purchase a newer model iPhone, as your iPhone 6s cannot be updated beyond iOS 15.