You need to follow exactly the instructions here. However, they are not really complete because although they say:
You will need to install the Roon 1.8 legacy app on a Roon Remote/iOS device first before being able to take the downgrade
what they don’t say is that you must keep the legacy Roon remote open while carrying out the downgrade to 1.8 on the Nucleus. I had a number of futile downgrade attempts until I appreciated that was what was needed. After I did that the downgrade worked.
Stephen