How can I update my iPhone 6s to iOS 15?

An iPhone 5s, iPhone 6, or iPhone 6 Plus isn't capable of 13 or above; they require at least 2GB of RAM. If you're referring to an iPhone 6s, use a computer or the Software Update function in Settings.

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If you are referring to an iPhone 6 as stated in your tagline, it can only be updated to iOS 12.5.7. If you are referring to an iPhone 6s, then it can be updated to iOS 15.8.3, and no further due to hardware limitations. Either way, it’s time to start looking into a newer iPhone that can run a newer iOS.

You can't. It's too old for iOS 15.