Ok so I have raised a ticket and this is what support came back with.

I’m sorry you’re having problems. Below are some troubleshooting suggestions for getting the update to work.

1. Use Recovery Mode.

Connect the provided USB-C cable from your H90 to your computer.
Press and hold the Select Knob, Perform Knob, and Quick Knob 1, while inserting the power cable to enter updater mode. The screen will say “Recovery Mode”.
Open H90 Control and click “Connect”.
Follow the on-screen instructions to update your H90.
2. Make sure you are not using a USB hub. 

3. Make sure you are using the Eventide power supply. 

4. Make sure there are no other USB or MIDI devices attached to your computer.

5. Try a different USB cable

6. Try a different USB port.

7. Make sure no other programs are running.

8. Make sure antivirus or firewall programs are not interfering with the update.

9. Restart your computer.

10. Try a different computer.

None of this worked for me, so I have gone back to support requested further help – nothing received so far. This is clearly an issue with their upgrade package and needs fixing.