How can I update my iPhone 6s Plus to iOS 13?

An iPhone 6 Plus isn't capable of 13 or above.

An iPhone 6s Plus can only be updated directly to 15; to do this, use a computer or the Software Update function in Settings.

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The iPhone 6 Plus does not have the required hardware to update to iOS 13 or higher.

An iPhone 6 cannot be upgraded past its current 12.5.5. A 6s or later is required.