It is best to wait until your return is accepted before amending your return. If it is rejected for any reason, you can make the changes when the return comes back. If the return is accepted, you can go in and make the change in TurboTax. There is no fee for the amendment but you will need to mail in the amendment to the address which will print with the return, as the IRS will not accept them as an e-file.
Here is a link to check the status of your TurboTax e-file.
Here is a TurboTax FAQ with step-by-step instructions for amending your return.