How can I update my iPhone

Assuming you have a compatible device, see: Update your iPhone, iPad, or iPod touch - Apple Support

And: If your iPhone, iPad, or iPod touch won't update - Apple Support

If the above Apple articles do not help, please post back with relevant information. 

NOTE: You can not update to iOS 16.

An iPhone 6s isn't capable of 16 or above; its CPU isn't powerful enough. To install 15.7.3, use a computer or the Software Update function in Settings.

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