How to change system updaute from WiFi to mobile network

You misunderstand; you need to be connected to a Wi-Fi network to download the update.

You cannot download it over a cellular data connection (at the request of the cellular providers, it's not a restriction Apple decided to impose.)

Following on from Dogcow-Moof's answer. Only iPhones that support 5G can be updated via Mobile Data.

• 5G = iPhone SE 3. Gen, iPhone 12, 13 and 14 series

But out can also update from a PC/Mac.

How to update via iTunes on Mac or Windows:

• Update your iPhone, iPad, or iPod touch with iTunes - Apple Support

To update via Finder on Mac:

• Update your iPhone, iPad, or iPod touch with the Finder - Apple Support