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
If you used the TurboTax online editions to complete and file the 2018 tax return -
To access your current or prior year online tax returns sign onto the TurboTax website with the User ID you used to create the account - https://myturbotax.intuit.com/
Scroll down to the bottom of the screen and on the section Your tax returns & documents click on Show. Click on 2018 and Click on Amend (change) return and Click on Download .tax file
NOTE - If you paid for one of the TurboTax 2018 online editions then contact TurboTax support and they can push a 2018 desktop installation file to your download account at no charge - https://support.turbotax.intuit.com/contact/
Otherwise you will need to purchase and download one of the 2018 desktop editions from this website to amend the 2018 tax return - https://turbotax.intuit.com/personal-taxes/past-years-products/