You won't be able to amend your tax return at this time because the IRS hasn't issued the 1040-X Amendment form. The IRS estimate for issuing the 1040X form is February 14, 2019.
Please see the following TurboTax FAQ instructions on how to amend your tax return once the IRS issues the 2018 form: https://ttlc.intuit.com/replies/3288565
[Edited 2-3-19|5:08 am PST]
