Tax Returns do not appear in "my documents". The Amend button will be available in TurboTax today. Click Amend and follow the directions to amend and then e-file the amend.
My documents is a place where you can upload your personal documents, receipts, and other forms to save with your current year's tax information.
Previous tax years are accessed by clicking Tax Home and then Reports and Documents at the bottom of the screen. Amend just became available in TurboTax in the last two days, so that is why you did not see it before.
How to Amend