Can you use a iPad to update a iPhone if the phone does not have enough storage

No. Sorry.

This would work Though.

”Update your device using your computer

If you can’t update wirelessly on your device, you can update manually using a computer that you trust. If your computer is using Personal Hotspot on the device that you’re updating, connect your computer to a different Wi-Fi or Ethernet network before you update.

1. On a Mac with macOS Catalina 10.15, open Finder. On a Mac with macOS Mojave 10.14 or earlier, or on a PC, open iTunes.
2. Connect your device to your computer.
3. Locate your device on your computer.
4. Click General or Settings, then click Check for Update. 
5. 
6. 
   
7. Click Download and Update.
8. If asked, enter your passcode. If you don't know your passcode, learn what to do.

“

No, to update the phone you need a Windows PC with iTunes or a Mac.

See→Update your iPhone, iPad, or iPod touch - Apple Support or watch this video: