On the phone, I assume.
Add the new account (in Accounts). Unless you're sure that the old account has been removed from the server (so people sending emails there get error messages that the account doesn't exist), or it's so flooded with spam that you don't care what comes to it, I'd leave the old account alone for a month or two. Then delete it from the phone, once you're sure that your notification that you won a million bucks won't be coming to that account.