See this TurboTax support FAQ for amending a 2018 tax return originally completed and filed using the online editions - https://ttlc.intuit.com/community/tax-topics/help/how-do-i-amend-my-2018-turbotax-online-return/01/9...
A Mac download is not required to amend a 2018 online tax return. You amend the 2018 return online.
