Use mobile network to software update

You can't unless have 5G which you won't have on an iPhone 7. You'll have to use wifi or a computer.

Click on a link below for full instructions about how to update with wi-fi or 5G (if using 5g then see: Use 5G with your iPhone https://support.apple.com/HT211828 ), or using a computer if you have storage or connectivity issues.

Update your iPhone, iPad, or iPod touch. Click here --> https://support.apple.com/HT204204

Or this video:

"How to update your iPhone or iPad" click here --> https://youtu.be/csJAEyvvruY