2018 (or earlier) cannot be amended online.
Amended returns can only be mailed. It is suggested that it be mailed certified with return receipt (or other tracking service) to verify that the IRS receives it.
See this TurboTax FAQ for detailed amend instructions:
https://ttlc.intuit.com/questions/1894381-how-to-amend-change-or-correct-a-return-you-already-filed
-- Amended returns can only be mailed - allow 8-12 weeks - can take up to 16 weeks (4 months) for processing.
You can check the status of the amended return here, but allow 3 weeks after mailing.
https://www.irs.gov/filing/wheres-my-amended-return
**Disclaimer: This post is for discussion purposes only and is NOT tax advice. The author takes no responsibility for the accuracy of any information in this post.**
