How to update mi iPhone with mobile data?

You can't; that didn't become available until the iPhone 12 and requires a 5G connection. If it matters, an iPhone 6 isn't capable of 13 or above.

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iPhone 6 with IOS 12 I am not aware or have not tried. But iPhone 8 and above with 4G is possible. Please see the pic below

Didn't you get the option as shown below when you started the update?

You will need an iPhone 12 and a 5G connection from your carrier to update using data.

You have two update options:

1) Wi-Fi

2) Direct connection to a Mac or PC