How do I cancel an amended tax file? I only wanted to amend it for the sole purpose of efiling instead of by mail.

Click on Cancel Amend on the left side of the screen.

See this TurboTax support FAQ for changing from mail to e-file - https://ttlc.intuit.com/community/e-file/help/how-do-i-change-from-mail-to-e-file-in-turbotax-online/01/26985

You can delete the amended return this way.  Do the steps slowly and carefully.  After you delete it then you need to manually take out the changes you made to put your return back to the original return.

How to delete an Amended return in the Online version

https://ttlc.intuit.com/community/amending/help/how-do-i-cancel-or-undo-an-amended-return-in-turbotax-online/00/26315

Were you trying to change your original from mailing to efile?

How to change from Mail to Efile in the Online version

https://ttlc.intuit.com/community/e-file/help/how-do-i-change-from-mail-to-e-file-in-turbotax-online/00/26985

Click on Add a State to let you back into your retun.

I have tried doing both of these steps that you had given and first off, I haven't seen a "cancel" button and don't know where I can "open my return if it hasn't been open already" at. This is very confusing and frustrating because I was trying to change from "by mail" to "e-file" and then it amended it and I don't know how to cancel it. I NEED TO RESOLVE THIS. 

Do you have the CD/Download version or the Online version?