You should be able to change the qualifying person under My Information, your filing status.  Edit and answer the filing status questions again.  You should be aware that, if you are divorced or legally separated, Head of Household can only be claimed by the parent with whom the child lived for more than 6 months.  If you're an unmarried couple living with the child who belongs to both of you, only the parent claiming the child as a dependent can claim HOH.