My husband is head of household and I'd like to add him as that in my old account. We've never filed through Turbo Tax together. How do we do that?

You can't.

If you want him listed first on your tax return, you'll have to start a new account and you will not be able to transfer anything over.

There is no such thing as Head of Household for a  married couple. Either one of you can be listed first on a tax return.

if it's acceptable to you, for you to be listed first, then you can use you existing account and add your husband as the spouse. Your old information will transfer over.

In IRS lingo, the first person listed is the taxpayer and the 2nd is the spouse. The "taxpayer' does not necessarily have to be the higher earner.