You need to restore your return to it's status at the time of filing. If you have a save .tax file, open that and start the amend process from there. If you don't have a saved copy of your return in that status, you will need to undo what you've done since filing, and then start the amended return.
For instructions on how to amend in TurboTax Desktop, see the following FAQ:
https://ttlc.intuit.com/replies/7276601
