How do I switch from Wi-Fi to mobile data to able to update to IOS 16

You must have 5G service with your iPhone 12 to be able to download an update over cellular data. If you do, then simply turn OFF Wi-Fi by opening Settings ➡️ W-Fi: Set this to OFF.

Once that is done then you will be on your 5G data service and you should be able to download and install the update over cellular data.

Axel F.

You will need 5G service if you want to update using cellular. Not sure though, why you want to do this if you have a good Wi-Fi connection available.

You need a reliable 5G connection to be able to download iOS 16 on cellular. The process for doing it is the same as it would be if you were using WiFi.