Is this amended tax return only at the stage of being worked on in TurboTax Online, or did you already print, sign, and mail it?
If you have mailed the amended return to the IRS (or state), there is no way to withdraw or cancel it. It is going to be processed. If you have made a mistake on the amended return, you will have to file another amendment, but I advise you to be patient before doing so. The IRS takes as long as 16 weeks to process amended returns; sending in a second amendment before the first is processed could just lead to confusion.
On the other hand, if you worked on the amendment online, but have taken no steps to file it, follow the steps below:
https://ttlc.intuit.com/replies/4170678