If a 2018 1099-R then you must amend 2017 to enter it. The box 2a amount is taxable in 2017 and will also have a 10% penalty if you are under age 59 1/2.
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.
https://ttlc.intuit.com/questions/1894381-how-to-amend-change-or-correct-a-return-you-already-filed
https://ttlc.intuit.com/replies/3288565
You can check the status of the amended return here, but allow 3 weeks after mailing.
https://www.irs.gov/filing/individuals/amended-returns-form-1040-x-/wheres-my-amended-return-1