If you have already filed the return, you will not be able to make changes to it. You will have to amend it. Do not amend until you have determined if the IRS accepted or rejected the return. If accepted wait until either you have received your refund or any payment you have made to the IRS has cleared. If rejected you may go ahead and amend.
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
02/26/2020 , 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_online_individual.html
How do I amend my 2019 TurboTax Online return?