How to update my iPhone 6s to IOS 13?

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

If you're referring to an iPhone 6s, use a computer or the Software Update function in the Settings. If you encounter an issue, describe it here.

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You cannot update to iOS 13 with an iPhone 6 or iPhone 6 Plus.

iOS 12.4.8 is the highest you can go.