"I need to make amend."

If you already filed a joint return for 2018 it is too late to amend the return and file married filing separately.  The rules do not allow you to change from a joint return to separate returns after the filing deadline, which was April 15, 2019.

while I agree it's too late to change, why would you want to do this in any event?  it's quite unusual that filing married- separate has a better outcome. what are the specific circumstances that cause you to want to do this? 

The poster never said anything about "Married filing Jointly" - the poster asked how NOT to file *Head of Household* which TurboTax will recommend if the unmarried (or considered unmarried) tax payer has a dependent and otherwise qualifies.

You can always change the filing status to force another filing status, but again - why would you want to not file HOH if you qualify for the larger standard deduction for HOH?

@jmarron310 - please clarify the original filing status on the first filed tax return.   If it actually was Married Filing Jointly then the other comments apply - it cannot be changed after April 15, 2019 unless you actually were not married at all, in which case you need professional help since that cannot be fixed by amending.

@macuser_22 You're right--I might have misinterpreted the original question.  We need clarity on what filing status has already been used for the 2018 return that @jmarron310 says needs to be amended.