Actually you do NOT amend. You only amend accepted and filed original returns. You just fix it and mail it in as the original return.
How to finish a prior year online return https://ttlc.intuit.com/community/prior-year-return/help/how-do-i-finish-a-prior-year-return-that-i-...
To access your current or prior year online tax returns sign onto the TurboTax website with the userID 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 the Year. Click on the Amend (change) return and Download .tax file
See this TurboTax support FAQ for amending a 2017 tax return originally completed and filed using TurboTax - https://ttlc.intuit.com/community/amending/help/how-do-i-amend-a-return-in-turbotax/01/27372