Can i download a form into a prior years tax return if it's already filed?

2019 and earlier can only be amended with TurboTax using the CD/download version installed on a PC or Mac computer.

2019 amended Federal returns can either be e-filed or mailed. 2018 and earlier 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 or state 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