Why can’t I update my phone using my data in phone

shifana999 Said:

"Why can’t I update my phone using data in the phone"

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Because your model is not made for it.

To Update Over Cellular:

You need an iPhone 12, that is using 5G in order to update over Cellular. Elsewise updating is not an option for your iPhone, when it comes through use of Cellular. So, you'd have to update over Wi-Fi, instead.

1. Go Here: Use 5G with your iPhone - Apple Support
2. Scroll to: Data Mode
3. Read: First two bullets

Important:

Back Up First: Be sure create a back up your iPhone prior to performing an update, that way you have something to restore your iPhone from, should anything go wrong afterwards.

Because the cellular providers don't permit that. You'll need to use Wifi or update your phone using your home computer with iTunes.