If you are legally married and living together in the same household at any time during the last six months of 2018 then you can NOT file as Head of Household, you can only file your tax return as either Married Filing Jointly or Married Filing Separately.
If that is your situation then you will have to amend the 2018 tax return to change your filing status.
See this TurboTax support FAQ for amending a tax return originally completed and filed using TurboTax - https://ttlc.intuit.com/community/amending/help/how-do-i-amend-a-return-in-turbotax/00/27579
