How do I update my iPhone 6s to iOS 18?

You cannot update your iPhone 6s to iOS 16 – let alone to iOS 18.

For that phone, which originally ran iOS 9 when Apple introduced the first variants nearly 10 years ago, iOS 15 is the end of the line.

An iPhone 6s, iPhone 6s Plus, iPhone 7, iPhone 7 Plus, or iPhone SE 1 isn't capable of 16 or above; they require an A11 or better CPU.

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Probably you are seeking to update becuase --> New Apps that require higher iOS versions than your device can be updated to... - Apple Community