iOS update w/o iTunes backup

1. Ensure iTunes is completely closed. (Use "Quit" in your dock.)

2. Launch the macOS Terminal application. This can be found using Finder, under Applications / Utilities / Terminal. Alternatively, it can be launched using Spotlight by pressing ⌘ Cmd +  and typing Terminal before pressing ⏎ Enter.

3. Instruct the Terminal to change disable automatic iTunes backups, by entering the following command before pressing ⏎ Enter:

defaults write com.apple.iTunes AutomaticDeviceBackupsDisabled -bool true

From this point on, if you open iTunes it will allow you to update your iPhone's version of iOS without automatically starting the backup process. If you'd like to undo this, follow the same steps 1 - 3, but run this command, instead:

defaults write com.apple.iTunes AutomaticDeviceBackupsDisabled -bool false

Source: https://reincubate.com/support/how-to/update-iphone-without-backup