Can I update my iPhone 6s to iOS 17?

iOS 17 will be released tomorrow for all phones, which are capable of running iOS 17. As the others have already told you, if you own an iPhone 6s, you don't own a phone capable of running iOS 17. So if you want iOS 17, then you'll need to buy a newer, more modern iPhone.

if you have: iPhone XR, XS, SE 2nd or 3rd edition, iPhone 11 and newer: You can update to iOS 17

If you have the iPhone 6S that appears in your profile you cannot update to iOS 17; your phone’s hardware cannot support any version higher than iOS 15.

Not on an iPhone 6s (which cannot support iOS versions past 15.7.9 because of hardware limits), and not before tomorrow, when the official iOS 17 is released.

Regards

Giulio