How can I download iOS 15.7.1

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 Settings.

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For an iPhone 6 you can not. That model can be updated to iOS 12.5.6.

To update to iOS 15.7.1, you will need an iPhone 6S or. 7.

Hello mahaity, For iPhone 6 the highest IOS update is 12.5.6. Any higher you will need a newer iPhone.