How to update iOS 15.0 ?

If you're referring to an iPhone 6 Plus, it's not capable of 13 or above.

If you're referring to an iPhone 6s Plus, use a computer or the Software Update function in Settings to install 15.8.

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Tap Settings > General > Software Update

If your phone is qualified for an update, it will appear.

The iPhone 6s Plus cannot update to iOS 16 or iOS 17, if that is what you are asking.