How can i update my iphone using mobile data?

You need WiFi or a computer. From the article below.

1. Plug your device into power and connect to the Internet with Wi-Fi.

Update your iOS device.        

Have you tried using iTunes on a computer or Finder if running Catalina to do the update?

 Update software on iOS devices in iTunes on Mac

Backing up to iCloud using cellular data is not possible. You will need to be connected to a WiFi network.

https://support.apple.com/en-us/HT203977