How can I update an iOS device using iTunes on Mac without backing it up?
You can update your device using Mobile Data by creating a WiFi Hotspot, while your data is on and update your device using the WiFi Hotspot. 

Try this:


  Quit iTunes.
  
  Open a Terminal (Applications >  Utilities > Terminal)
  
  Type or copy the following  command:defaults write  com.apple.iTunes 
  AutomaticDeviceBackupsDisabled - bool true
  
  Open iTunes.
  
  Connect the iPhone.
  
  Make Sync without backing up.



https://discussions.apple.com/thread/7640010?answerId=32668610022#32668610022