"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.