Please follow the instructions in the letter, as the IRS will handle this without an amendment. There should be a phone number on the notice, if you have already send in the amendment.
Turbo Tax can help you complete a Form 8962 if you have received a letter from the IRS asking for an update. Be sure to check if the letter references tax year 2017 or 2016. The form should have been generated when you entered your 1095-A into the Health Insurance section of Turbo Tax, but if for some reason it was not entered, these are the step-by-step instructions to add them.