If you received IRS Letter 12C, you will need to provide the IRS with the requested documentation so they can finish processing your return. Letter 12C typically requests the following items: 

• a copy of the Form 1095-A that you received showing your Marketplace health insurance coverage, 

• the completed Form 8962 that reconciles the advanced Premium Tax Credit (PTC) amounts.
  
  NOTE: You do NOT need to amend your return or submit Form 1040-X. Simply respond to the letter and attach the requested documentation. 

The IRS provides more detailed information about how to respond to Letter 12C in the Understanding Your Letter 12C page.

Also see the Form 8962 with Advance Premium Tax Credit - Repayment FAQ.

Note that any link in the information above is updated each year automatically and will take you to the most recent version of the webpage or document at the time it is accessed.