How can I use mobile data

That functionality was added with iOS 14.5 so the iPhone must be running at least that version or newer.

Settings > General > About > Software Update > Download and Install

You will get a prompt about the cellular download. Click on Continue.

Emmanuelagyapong Said:

"How can I use mobile data: Please can I use mobile data to update my iOS"

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Updating Over Cellular:

Yes. But you need an applicable model to do so. See my user tip: Use an iPhone 12, that is using 5G in order to update over Cellular - User Tip