How can I install iso 13 to my phone?

If you're referring to an iPhone 6s or newer, use a computer or the Software Update function in Settings to update it to 14.7.1; otherwise, it's incapable of 13 or above.

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You can't if you have an iPhone 5s, iPhone 6 and iPhone 6 Plus.

The hardware and RAM are insufficient, you need to buy a new(er) iPhone model to update.

If you have a newer iPhone model then only iOS 14.7.1 is available.