You will be able to
amend as soon as Form 1040X becomes available.
At this time the expected date of availability for the form 1040X is
2/28/19, however that date is subject to change. You can track it's availability by going to
the following link.
https://care-cdn.prodsupportsite.a.intuit.com/forms-availability/turbotax_fed_windows_individual.html