There should be a Cancel Amend in the left menu.

Otherwise, cancel by deleting Form 1040-X for the federal and the corresponding amended return for any state filing.

To see these forms, follow these steps:

1. From the left menu, select Tax Tools.
2. Select Tools.
3. Scroll to Delete a Form.
4. Scroll to Form 1040-X and select Delete and Delete Selected Form and Continue.
   1. Scroll to the state section and scan for the amended return form.
5. Scroll to the bottom of the next window and select Continue With My Return.

For more information, see: Do I need to amend?

10 Tax Tips for Filing an Amended Return - TurboTax Tax Tips & Videos.