How to update with mobile data?

You can't. The download won't initiate using only cellular signal. You can connect to Wifi to get the download, and then proceed with the install later off Wifi, but connect to a power source.

Alternatively, you can update by connecting to iTunes. Follow the instructions here: https://support.apple.com/en-us/HT204204

No. You have to use WiFi or use iTunes on a computer but you cannot update using cellular data.

You cannot.