If you need to attach Form 8833 to your tax return and it has already been processed and accepted by the IRS, then you will have to file an amended tax return, Form 1040X. Before amending the tax return wait for the IRS to send you the tax refund or if you have taxes owed then for the IRS to process the tax payment. Do not file your amended return before the IRS has processed your return and has any refund or balance due with the original return has been received and processed.
When you amend your return, you will not change anything in TurboTax. You just need to add an explanation that you are only amending the return to attach the Form 8833 which was not attached to the original return that was sent.
If you want you can print out Form 1040X and complete it by hand. Just make sure to attach your return and Form 8833 to the return before you mail it. You can also find the mailing address with the Form and Instructions as to where it should be mailed.
Form 1040X - https://www.irs.gov/pub/irs-pdf/f1040x.pdf
Form 1040X Instructions https://www.irs.gov/pub/irs-pdf/i1040x.pdf
