I had the same question. I have two houses and two HA installations. Only one was connected to Nabu Casa, which is the one we was sold. Therefore I wanted to detach that HA instance from Nabu Casa and connect the other one to the same account.
I sent a message asking the best practice about how to do this to Nabu Casa tech support (on a Sunday) and received an almost immediate response (great support). Their response, a very simple method, is pasted below:
" You would sign out of the instance at one house at your Configuration > Cloud page using the 3 dots in the upper right corner.
On the new instance you would associate it with the cloud service on the same page"