IRS Form 1040X, Amended U.S. Individual Income Tax Return is estimated to be available on 02/22/19
See this TurboTax support FAQ for IRS forms availability - https://ttlc.intuit.com/questions/1908854-irs-forms-availability-table-for-turbotax-individual-perso...
The tax return should not be amended until the tax refund has been received from the IRS.
See this TurboTax support FAQ for amending a tax return - https://ttlc.intuit.com/questions/1894381-how-to-amend-change-or-correct-a-return-you-already-filed