If your federal tax return has been filed and accepted by the IRS, then you can only amend the tax return to make any additions or changes. The 2018 amendment program will not be available until sometime in December when the 2019 online program goes live.
An amended federal tax return, Form 1040X, can only be printed and mailed, it cannot be e-filed. The IRS will take up to 4 months or longer to process the amended tax return. If taxes are owed after completing the amended return, you send a check with the amended return when it is mailed. If the amended return has a tax refund, then the IRS will send you a refund check in the mail for the refund, the refund cannot be direct deposited.
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
forgot to include it in 2018 return follow Critters instructions.
included in return but forgot to attach it to mailed return . hold on until irs contacts you, if at all
efiled and included in return. nothing to send and the IRS won't contact you for it.