How do I update iPhone without computers

A link should not be needed.

Settings > General > Software Update

If an update is available, it will appear. Make sure you have a good Wi-Fi connection and fully charged battery.

If you really do want a link........Update your iPhone or iPad - Apple Support

Best you can do on an iPhone 6 Plus is iOS 12.5.7

An iPhone 6 Plus isn't capable of 13 or above. To install 12.5.7, use the Software Update function in Settings; if you encounter a specific issue, describe it in this thread.

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