If you are using the TurboTax desktop editions you need to Continue the tax return that needs to be amended.
After Continuing the tax return click on Federal Taxes
Then click on Other Tax Situations
Under Other Tax Forms
On Amend a return, click on the start or update button
If you are using the TurboTax online editions -
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 the Year and Click on Amend (change) return