On 2018 Return, filed in 2019, I deducted IRA amount. However, I failed to fund the IRA for 2018. How do I amend the 2018 Tax Return?

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


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