If you filed the original tax return with TurboTax there is no additional charge to file a Form 1040X amended return.
Note that Form 1040X cannot be e-filed. You have to print it, sign it, and file it by mail. It takes the IRS up to 16 weeks to process an amended return.
Don't file the amended return until your first return has been accepted and you have received the original refund.
