You would have to amend your return for the relevant year. Since Turbotax does not support nonresident alien returns, you would have to download the forms from the IRS website and fill them out yourself (or use a tax professional).
You would need form 1040NR or 1040NR-EZ plus form 1040X for the amendment. The percentage would depend on your taxable income. What state are you if? Usually, when states recognize treaty exemption, which are federal, the same rules apply, so you most likely have to pay state taxes as well. You will also be subject to interest/underpayment penalty. So do this as soon as possible.
