See this TurboTax support FAQ for amending a 2018 tax return originally completed and filed using TurboTax - https://ttlc.intuit.com/community/amending/help/how-do-i-amend-a-return-in-turbotax/01/27579
The instructions for amending the online return are incorrect in the above FAQ. Follow the instruction is this FAQ and substitute 2018 for 2017 - https://ttlc.intuit.com/community/amending/help/how-do-i-amend-my-turbotax-online-return/00/27371
