I started to amend my 2018 return then discovered that it did need to be amended how do I unamend the return?

The FAQ at the link below gives instructions for canceling an amended return. But I think that will just remove the Form 1040X. I don't think it will undo the changes that you made. You have to go back to your tax return and change it back to the way it was before you started to amend it.

If you used TurboTax Online:

https://ttlc.intuit.com/questions/2552802-how-do-i-cancel-or-undo-an-amended-return-in-turbotax-onli...

If you used the CD/Download TurboTax software:

https://ttlc.intuit.com/questions/2552833-how-do-i-cancel-or-undo-an-amended-return-in-the-turbotax-...