How can I update my iPhone 6s to iOS 15.0

It's no longer possible to update to 15.0. Use a computer or the Software Update function in Settings to install 15.7.8.

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To update to the most recent, compatible version, please 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.