I don't think you mean HCTC. Health insurance from the Marketplace was not eligible for the HCTC. You are probably thinking of the Premium Tax Credit (PTC), which is the "subsidy" for Marketplace insurance. If you did not claim the PTC on the tax returns that you filed, when you amend your tax returns enter the Form 1095-A that you got from the Marketplace. TurboTax will then fill out Form 8962 and calculate the PTC. Go to the link that DoninGA provided for instructions to amend your tax returns.
