Why can’t I update my phone

Is your tag line correct in that your iPhone 6s has not been updated since iOS 10? If yes, you have skipped several years of updates and if so, the best way to update your iPhone would be to use iTunes on your computer to do the udpate.

This link will explain how to do that --> Update your iPhone, iPad, or iPod touch with iTunes - Apple Support

Also note: You came here from an article about iOS 16.6.1. If you own an iPhone 6s the highest iOS version it can support is iOS 15.7.8

And what error message appears when you try to run an update?

A screenshot would be really helpful, if you can.

If your iPhone or iPad won't update - Apple SupportApple SupportOfficial Apple Support › en-us

Hello,

The iPhone 6S can only be updated to 15.7.7 See this:

Updating iOS: common issues and solutions - Apple Community

Good luck.