How to update my iPhone 6s to iOS 16?

Update your iPhone, iPad, or iPod touch

• If you have an iPhone 5S or 6/6plus, the highest you can update to is 12.5.7

• If you have an iPhone 6S through 7, or SE 1st generation, you can update to 15.8.2

• If you have an iPhone 8 or X, you can update to 16.7.7

• If you have an iPhone SE (2nd generation or later), or XR through 15, you can update to 17.5

Go to Settings > General > Software Update.

This link will show you how to update your iPhone, iPad, or iPod touch using WiFi -

Update your iPhone or iPad - Apple Support

This link will show you how to update using your computer -

Update iOS on iPhone - Apple Support

Your Phone is to old but to update any Other update go to Settings then to General then to software update

It seems you're interested in updating your iPhone to iOS 16. Is there a specific reason you'd prefer iOS 16 over other versions, even though it might not be compatible with your device? Perhaps you're encountering limitations with apps that require a newer iOS version.

Apps that require higher iOS versions than your device can be updated to... - Apple Community