If there was not a fax number on the IRS letter you received you will have to print and mail a copy of the Form 1095-A and the Form 8962, Premium Tax Credit.
IRS website for Reconciling Your Advance Payments of the Premium Tax Credit - https://www.irs.gov/individuals/reconciling-your-advance-payments-of-the-premium-tax-credit
TurboTax support FAQ for IRS Letter 12C - https://ttlc.intuit.com/turbotax-support/en-us/help-article/tax-forms/got-12c-letter-irs-form-8962/L...
